Semiconductor device and display device including the same

ABSTRACT

To improve field-effect mobility and reliability in a transistor including an oxide semiconductor film. A semiconductor device includes a transistor including an oxide semiconductor film. The transistor includes a region where the maximum value of field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 40 and smaller than 150; a region where the threshold voltage is higher than or equal to minus 1 V and lower than or equal to 1 V; and a region where the S value is smaller than 0.3 V/decade.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/963,141, filed Apr. 26, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/464,534, filed Mar. 21, 2017, now U.S. Pat.No. 9,960,190, which claims the benefit of foreign priority applicationsfiled in Japan on Mar. 22, 2016, as Serial Nos. 2016-057718,2016-057720, and 2016-057716, all of which are incorporated byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor film and a display deviceincluding the semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. The present inventionrelates to a process, a machine, manufacture, or a composition ofmatter. One embodiment of the present invention particularly relates toan oxide semiconductor or a manufacturing method of the oxidesemiconductor. One embodiment of the present invention relates to asemiconductor device, a display device, a light-emitting device, a powerstorage device, a storage device, a driving method thereof, and amanufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach one embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

BACKGROUND ART

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a field-effect transistor (FET) or a thinfilm transistor (TFT)). Such a transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (display device). A semiconductor material typified bysilicon is widely known as a material for a semiconductor thin film thatcan be used in a transistor. As another material, an oxide semiconductorhas been attracting attention. For example, a technique in which atransistor is fabricated using an In—Ga—Zn-based oxide semiconductor isdisclosed (see Patent Document 1).

Furthermore, a semiconductor device achieving high field-effect mobility(simply referred to as mobility or μ_(FE) in some cases) with such astructure that a plurality of oxide semiconductor layers are stacked,the oxide semiconductor layers functioning as a channel in the pluralityof oxide semiconductor layers contains indium and gallium, and theproportion of indium is higher than the proportion of gallium isdisclosed (see Patent Document 1).

Non-Patent Document 1 discloses a homologous series represented byIn_(1−x)Ga_(1+x)O₃(ZnO)_(m) (−1≤x≤1, and m is a natural number).Furthermore, Non-Patent Document 1 discloses a solid solution range of ahomologous series. For example, in the solid solution range of thehomologous series in the case where m is 1, x ranges from −0.33 to 0.08,and in the solid solution range of the homologous series in the casewhere m is 2, x ranges from −0.68 to 0.32.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2007-96055-   [Patent Document 2] Japanese Published Patent Application No.    2014-007399

Non-Patent Document

-   [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.,” J.    Solid State Chem., 1991, Vol. 93, pp. 298-315.

DISCLOSURE OF INVENTION

The field-effect mobility of a transistor that uses an oxidesemiconductor film as a channel region is preferably as high aspossible. However, when the field-effect mobility is increased, thetransistor has a problem with its characteristics, that is, thetransistor tends to be normally on. Note that “normally on” means astate where a channel exists without application of a voltage to a gateelectrode and current flows through the transistor.

Furthermore, in a transistor that uses an oxide semiconductor film in achannel region, oxygen vacancies which are formed in the oxidesemiconductor film adversely affect the transistor characteristics. Forexample, oxygen vacancies formed in the oxide semiconductor film arebonded with hydrogen to serve as carrier supply sources. The carriersupply sources generated in the oxide semiconductor film cause a changein the electrical characteristics, typically, shift in the thresholdvoltage, of the transistor including the oxide semiconductor film.

When the amount of oxygen vacancies in the oxide semiconductor film istoo large, for example, the threshold voltage of the transistor isshifted in the negative direction, and the transistor has normally-oncharacteristics. Thus, especially in the channel region of the oxidesemiconductor film, the amount of oxygen vacancies is preferably smallor the amount with which the normally-on characteristics are notexhibited.

In addition, Non-Patent Document 1 discloses an example ofIn_(x)Zn_(y)Ga_(z)O_(w), and when x, y, and z are set so that acomposition in the neighborhood of ZnGa₂O₄ is obtained, i.e., x, y, andz are close to 0, 1, and 2, respectively, a spinel crystal structure isformed or is likely to be mixed. A compound represented by AB₂O₄ (A andB are metal) is known as a compound having a spinel crystal structure.

However, when a spinel crystal structure is formed or mixed in anIn—Ga—Zn-based oxide semiconductor, electrical characteristics orreliability of a semiconductor device (e.g., a transistor) including theIn—Ga—Zn-based oxide semiconductor is adversely affected by the spinelcrystal structure in some cases.

In view of the foregoing problems, an object of one embodiment of thepresent invention is to improve field-effect mobility and reliability ina transistor including an oxide semiconductor film. An object of oneembodiment of the present invention is to prevent a change in electricalcharacteristics of a transistor including an oxide semiconductor filmand to improve the reliability of the transistor. An object of oneembodiment of the present invention is to provide a semiconductor devicewith low power consumption. An object of one embodiment of the presentinvention is to provide a semiconductor device with favorable electricalcharacteristics. An object of one embodiment of the present invention isto provide a novel oxide semiconductor. An object of one embodiment ofthe present invention is to provide a novel semiconductor device. Anobject of one embodiment of the present invention is to provide a noveldisplay device.

Note that the description of the above object does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding a transistor including an insulating film, a first conductivefilm, a second conductive film, a third conductive film, and an oxidesemiconductor film. The first conductive film includes a region incontact with the oxide semiconductor film. The second conductive filmincludes a region in contact with the oxide semiconductor film. Thethird conductive film includes a region that overlaps with the oxidesemiconductor film with the insulating film therebetween. The transistorincludes a region where a maximum value of field-effect mobility of thetransistor at a gate voltage of higher than 0 V and lower than or equalto 10 V is larger than or equal to 40 cm²/Vs and smaller than 150cm²/Vs, a region where threshold voltage is higher than or equal to −1 Vand lower than or equal to 1 V, a region where an S value is smallerthan 0.3 V/decade, and a region where off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 1 and smaller than 1.5 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents a value of the field-effect mobility of the transistor at agate voltage of 2 V.

One embodiment of the present invention is a semiconductor deviceincluding a transistor including a first gate electrode, a firstinsulating film over the first gate electrode, an oxide semiconductorfilm over the first insulating film, a second insulating film over theoxide semiconductor film, a second gate electrode over the secondinsulating film, and a third insulating film over the oxidesemiconductor film and the second gate electrode. The oxidesemiconductor film includes a channel region overlapping with the gateelectrode, a source region in contact with the third insulating film,and a drain region in contact with the third insulating film. The firstgate electrode and the second gate electrode are electrically connectedto each other. The transistor includes a region where a maximum value offield-effect mobility of the transistor at a gate voltage of higher than0 V and lower than or equal to 10 V is larger than or equal to 40 cm²/Vsand smaller than 150 cm²/Vs, a region where threshold voltage is higherthan or equal to −1 V and lower than or equal to 1 V, a region where anS value is smaller than 0.3 V/decade, and a region where off-statecurrent is lower than 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) islarger than or equal to 1 and smaller than 1.5 where μ_(FE)(max)represents the maximum value of the field-effect mobility of thetransistor and μ_(FE)(V_(g)=2V) represents a value of the field-effectmobility of the transistor at a gate voltage of 2 V.

In the above embodiments, it is preferable that the oxide semiconductorfilm include a region where density of shallow defect states is lowerthan 1.0×10⁻¹² cm².

One embodiment of the present invention is a semiconductor deviceincluding a transistor including an insulating film, a first conductivefilm, a second conductive film, a third conductive film, and an oxidesemiconductor film. The first conductive film includes a region incontact with the oxide semiconductor film. The second conductive filmincludes a region in contact with the oxide semiconductor film. Thethird conductive film includes a region that overlaps with the oxidesemiconductor film with the insulating film therebetween. The transistorincludes a region where a maximum value of field-effect mobility of thetransistor at a gate voltage of higher than 0 V and lower than or equalto 10 V is larger than or equal to 40 cm²/Vs and smaller than 150cm²/Vs, a region where threshold voltage is higher than or equal to −1 Vand lower than or equal to 1 V, a region where an S value is smallerthan 0.3 V/decade, and a region where off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 1.5 and smaller than 3 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents a value of the field-effect mobility of the transistor at agate voltage of 2 V.

One embodiment of the present invention is a semiconductor deviceincluding a transistor including a first gate electrode, a firstinsulating film over the first gate electrode, an oxide semiconductorfilm over the first insulating film, a second insulating film over theoxide semiconductor film, a second gate electrode over the secondinsulating film, and a third insulating film over the oxidesemiconductor film and the second gate electrode. The oxidesemiconductor film includes a channel region overlapping with the gateelectrode, a source region in contact with the third insulating film,and a drain region in contact with the third insulating film. The firstgate electrode and the second gate electrode are electrically connectedto each other. The transistor includes a region where a maximum value offield-effect mobility of the transistor at a gate voltage of higher than0 V and lower than or equal to 10 V is larger than or equal to 40 cm²/Vsand smaller than 150 cm²/Vs, a region where threshold voltage is higherthan or equal to −1 V and lower than or equal to 1 V, a region where anS value is smaller than 0.3 V/decade, and a region where off-statecurrent is lower than 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) islarger than or equal to 1.5 and smaller than 3 where μ_(FE)(max)represents the maximum value of the field-effect mobility of thetransistor and μ_(FE)(V_(g)=2V) represents a value of the field-effectmobility of the transistor at a gate voltage of 2 V.

In the above embodiments, it is preferable that the oxide semiconductorfilm include a region where density of shallow defect states is higherthan or equal to 1.0×10⁻¹² cm² and lower than 2.0×10⁻¹² cm².

One embodiment of the present invention is a semiconductor deviceincluding a transistor including an insulating film, a first conductivefilm, a second conductive film, a third conductive film, and an oxidesemiconductor film. The first conductive film includes a region incontact with the oxide semiconductor film. The second conductive filmincludes a region in contact with the oxide semiconductor film. Thethird conductive film includes a region that overlaps with the oxidesemiconductor film with the insulating film therebetween. The transistorincludes a region where a maximum value of field-effect mobility of thetransistor at a gate voltage of higher than 0 V and lower than or equalto 10 V is larger than or equal to 10 cm²/Vs and smaller than 100cm²/Vs, a region where threshold voltage is higher than or equal to −1 Vand lower than or equal to 1 V, a region where an S value is smallerthan 0.3 V/decade, and a region where off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 3 and smaller than 10 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents a value of the field-effect mobility of the transistor at agate voltage of 2 V.

One embodiment of the present invention is a semiconductor deviceincluding a transistor including a first gate electrode, a firstinsulating film over the first gate electrode, an oxide semiconductorfilm over the first insulating film, a second insulating film over theoxide semiconductor film, a second gate electrode over the secondinsulating film, and a third insulating film over the oxidesemiconductor film and the second gate electrode. The oxidesemiconductor film includes a channel region overlapping with the gateelectrode, a source region in contact with the third insulating film,and a drain region in contact with the third insulating film. The firstgate electrode and the second gate electrode are electrically connectedto each other. The transistor includes a region where a maximum value offield-effect mobility of the transistor at a gate voltage of higher than0 V and lower than or equal to 10 V is larger than or equal to 10 cm²/Vsand smaller than 100 cm²/Vs, a region where threshold voltage is higherthan or equal to −1 V and lower than or equal to 1 V, a region where anS value is smaller than 0.3 V/decade, and a region where off-statecurrent is lower than 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) islarger than or equal to 3 and smaller than 10 where μ_(FE)(max)represents the maximum value of the field-effect mobility of thetransistor and μ_(FE)(V_(g)=2V) represents a value of the field-effectmobility of the transistor at a gate voltage of 2 V.

In the above embodiments, it is preferable that the oxide semiconductorfilm include a region where density of shallow defect states is higherthan or equal to 2.0×10⁻¹² cm² and lower than 3.0×10⁻¹² cm².

In the above embodiments, it is preferable that the oxide semiconductorfilm include a composite oxide semiconductor in which a first region anda second region are mixed; the first region include a plurality of firstclusters including, as a main component, one or more selected fromindium, zinc, and oxygen; the second region include a plurality ofsecond clusters including, as a main component, one or more selectedfrom indium, an element M, zinc, and oxygen; the element M be Al, Ga, Y,or Sn; the first region include a portion in which the plurality offirst clusters are connected to each other; and the second regioninclude a portion in which the plurality of second clusters areconnected to each other.

In the above embodiments, it is preferable that an atomic ratio of theindium to the element M and the zinc be In:M:Zn=4:2:3 or in aneighborhood of 4:2:3, and, when the In is 4, the element M be greaterthan or equal to 1.5 and less than or equal to 2.5 and the Zn be greaterthan or equal to 2 and less than or equal to 4. In the aboveembodiments, it is preferable that an atomic ratio of the indium to theelement M and the zinc be In:M:Zn=5:1:6 or in a neighborhood of 5:1:6,and, when the In is 5, the element M be greater than or equal to 0.5 andless than or equal to 1.5 and the Zn be greater than or equal to 5 andless than or equal to 7.

In the above embodiments, it is preferable that the first clusters haveelectrical conductivity and the second clusters have electricalsemiconductivity.

In the above embodiments, it is preferable that the first clusters eachinclude a portion longer than or equal to 0.5 nm and shorter than orequal to 1.5 nm.

One embodiment of the present invention is a display device including adisplay element and the semiconductor device of any one of the aboveembodiments. One embodiment of the present invention is a display moduleincluding the display device and a touch sensor. One embodiment of thepresent invention is an electronic device including the semiconductordevice of any one of the above embodiments, the display device, or thedisplay module; and an operation key or a battery. One embodiment of thepresent invention is an electronic device including the semiconductordevice of any one of the above embodiments; and an inverter or aconverter.

One embodiment of the present invention can improve field-effectmobility and reliability in a transistor including an oxidesemiconductor film. One embodiment of the present invention can preventa change in electrical characteristics of a transistor including anoxide semiconductor film and improve the reliability of the transistor.One embodiment of the present invention can provide a semiconductordevice with low power consumption. One embodiment of the presentinvention can provide a semiconductor device with favorable electricalcharacteristics. One embodiment of the present invention can provide anovel oxide semiconductor. One embodiment of the present invention canprovide a novel semiconductor device. One embodiment of the presentinvention can provide a novel display device.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows I_(d)−V_(g) characteristics of a transistor.

FIG. 2 shows I_(d)−V_(g) characteristics of a transistor.

FIG. 3 shows I_(d)−V_(g) characteristics of a transistor.

FIGS. 4A and 4B are a schematic top view and a schematic cross-sectionalview illustrating a composite oxide semiconductor.

FIGS. 5A and 5B are a schematic top view and a schematic cross-sectionalview illustrating a composite oxide semiconductor.

FIGS. 6A and 6B are a schematic top view and a schematic cross-sectionalview illustrating a composite oxide semiconductor.

FIGS. 7A and 7B are a schematic top view and a schematic cross-sectionalview illustrating a composite oxide semiconductor.

FIG. 8 illustrates an atomic ratio of an oxide semiconductor.

FIGS. 9A and 9B illustrate a sputtering apparatus.

FIG. 10 is a process flow chart showing a method for manufacturing acomposite oxide semiconductor.

FIGS. 11A and 11B show a cross section of the vicinity of a target.

FIGS. 12A and 12B show a cross section of the vicinity of a target.

FIG. 13 shows HAADF-STEM observations.

FIGS. 14A and 14B show I_(d)−V_(g) characteristics and I_(d)−V_(d)characteristics of a transistor.

FIG. 15 shows I_(d)−V_(g) characteristics and linear and saturationmobility curves which are calculated on the basis of GCA.

FIG. 16 shows I_(d)−V_(g) characteristics and field-effect mobilitycurves of a FET including a CAAC-OS.

FIG. 17A is a top view of a transistor, and FIGS. 17B and 17C arecross-sectional views thereof.

FIG. 18 is a schematic view illustrating a concept of an effectivechannel length of a transistor.

FIGS. 19A to 19C are schematic views each illustrating a donor density.

FIG. 20 shows I_(d)−V_(g) characteristics.

FIG. 21 shows I_(d)−V_(g) characteristics.

FIG. 22 shows calculation results of a density of interface states.

FIGS. 23A and 23B show I_(d)−V_(g) characteristics.

FIG. 24 shows shapes of mobility curves.

FIG. 25 is a schematic view showing the contribution of drift currentand diffusion current in I_(d)−V_(g) characteristics.

FIG. 26 shows I_(d)−V_(g) characteristics and mobility curves of BGTCdual-gate FETs each including a CAAC-OS where sDOS is not assumed.

FIG. 27 is a band diagram in the thickness direction.

FIG. 28 shows I_(d)−V_(g) characteristics and mobility curves of BGTCdual-gate FETs each including a CAAC-OS where sDOS is assumed.

FIG. 29 shows the dependence of field-effect mobility (maximum) on anIGZO film thickness.

FIGS. 30A and 30B are graphs illustrating sDOS distribution in IGZOfilms and shapes of mobility curves.

FIG. 31A is a circuit diagram illustrating resistances and a FET, andFIG. 31B is a graph illustrating the relation between field-effectmobility and source and drain regions formed by reducing the resistanceof a channel region.

FIG. 32 is a graph illustrating saturation mobility when mobilitydepends on temperature.

FIG. 33 shows saturation-mobility curves of FETs.

FIG. 34 is a graph illustrating the influence of a reduction ineffective channel length on the shape of a mobility curve.

FIGS. 35A to 35C show mobility curves obtained by device simulationunder different conditions.

FIGS. 36A and 36B are cross-sectional views illustrating a semiconductordevice.

FIGS. 37A and 37B are cross-sectional views illustrating a semiconductordevice.

FIGS. 38A and 38B are cross-sectional views illustrating a semiconductordevice.

FIGS. 39A and 39B are cross-sectional views illustrating a semiconductordevice.

FIGS. 40A and 40B are cross-sectional views illustrating a semiconductordevice.

FIGS. 41A and 41B are cross-sectional views illustrating a semiconductordevice.

FIGS. 42A and 42B are cross-sectional views illustrating a semiconductordevice.

FIGS. 43A and 43B are cross-sectional views illustrating a semiconductordevice.

FIGS. 44A to 44C show band structures.

FIGS. 45A to 45C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 46A to 46C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 47A to 47C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 48A to 48C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 49A and 49B are cross-sectional views illustrating a semiconductordevice.

FIGS. 50A and 50B are cross-sectional views illustrating a semiconductordevice.

FIGS. 51A to 51C are a top view and cross-sectional views illustrating asemiconductor device.

FIG. 52 is a top view illustrating one embodiment of a display device.

FIG. 53 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 54 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 55 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 56 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 57 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 58A to 58D are cross-sectional views illustrating a method forforming an EL layer.

FIG. 59 is a conceptual diagram illustrating a droplet dischargeapparatus.

FIGS. 60A to 60C are a block diagram and circuit diagrams illustrating adisplay device.

FIG. 61 illustrates a display module.

FIGS. 62A to 62E illustrate electronic devices.

FIGS. 63A to 63G illustrate electronic devices.

FIGS. 64A to 64E illustrate electronic devices.

FIGS. 65A and 65B are perspective views illustrating a display device.

FIGS. 66A and 66B are perspective views illustrating a display device.

FIGS. 67A and 67B are each a circuit diagram of a semiconductor deviceof an embodiment.

FIG. 68 illustrates a cross-sectional structure of a semiconductordevice of an embodiment.

FIG. 69 illustrates a cross-sectional structure of a semiconductordevice of an embodiment.

FIG. 70 is a circuit diagram illustrating a memory device of oneembodiment of the present invention.

FIG. 71 is a circuit diagram illustrating a memory device of oneembodiment of the present invention.

FIGS. 72A to 72C are circuit diagrams and a timing chart illustratingone embodiment of the present invention.

FIGS. 73A to 73C are a graph and circuit diagrams illustrating oneembodiment of the present invention.

FIGS. 74A and 74B are a circuit diagram and a timing chart illustratingone embodiment of the present invention.

FIGS. 75A and 75B are a circuit diagram and a timing chart illustratingone embodiment of the present invention.

FIGS. 76A to 76E are a block diagram, circuit diagrams, and waveformdiagrams illustrating one embodiment of the present invention.

FIGS. 77A and 77B are a circuit diagram and a timing chart illustratingone embodiment of the present invention.

FIGS. 78A and 78B are each a circuit diagram illustrating one embodimentof the present invention.

FIGS. 79A to 79C are each a circuit diagram illustrating one embodimentof the present invention.

FIGS. 80A and 80B are each a circuit diagram illustrating one embodimentof the present invention.

FIGS. 81A to 81C are each a circuit diagram illustrating one embodimentof the present invention.

FIGS. 82A and 82B are each a circuit diagram illustrating one embodimentof the present invention.

FIG. 83 is a block diagram illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 84 is a circuit diagram illustrating a semiconductor device of oneembodiment of the present invention.

FIGS. 85A and 85B are top views illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 86A and 86B are a flowchart and a perspective view of asemiconductor device illustrating one embodiment of the presentinvention.

FIGS. 87A to 87C are perspective views illustrating electronic devicesof one embodiment of the present invention.

FIGS. 88A to 88E show a HAADF-STEM image and EDX mapping images of aplane.

FIGS. 89A to 89E show a HAADF-STEM image and EDX mapping images of across section.

FIG. 90 shows I_(d)−V_(g) characteristics of a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented in many different modes, andit will be readily appreciated by those skilled in the art that modesand details thereof can be changed in various ways without departingfrom the spirit and scope of the present invention. Thus, the presentinvention should not be interpreted as being limited to the followingdescription of the embodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over”, “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which the componentsare described. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow between thedrain and the source through the channel region. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” include a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

In this specification and the like, the term “parallel” means that theangle formed between two straight lines is greater than or equal to −10°and less than or equal to 10°, and accordingly also covers the casewhere the angle is greater than or equal to −5° and less than or equalto 50. The term “perpendicular” means that the angle formed between twostraight lines is greater than or equal to 800 and less than or equal to1000, and accordingly also covers the case where the angle is greaterthan or equal to 850 and less than or equal to 950.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

Unless otherwise specified, the off-state current in this specificationand the like refers to a drain current of a transistor in an off state(also referred to as non-conduction state and cutoff state). Unlessotherwise specified, the off state of an n-channel transistor means thata voltage (V_(gs)) between its gate and source is lower than thethreshold voltage (V_(th)), and the off state of a p-channel transistormeans that the gate-source voltage V_(gs) is higher than the thresholdvoltage V_(th). For example, the off-state current of an n-channeltransistor sometimes refers to a drain current that flows when thegate-source voltage V_(gs) is lower than the threshold voltage V_(th).

The off-state current of a transistor depends on V_(gs) in some cases.Thus, “the off-state current of a transistor is lower than or equal toI” may mean “there is V_(gs) with which the off-state current of thetransistor becomes lower than or equal to I”. Furthermore, “theoff-state current of a transistor” means “the off-state current in anoff state at predetermined V_(gs)”, “the off-state current in an offstate at V_(gs) in a predetermined range”, “the off-state current in anoff state at V_(gs) with which sufficiently reduced off-state current isobtained”, or the like.

As an example, the assumption is made of an n-channel transistor wherethe threshold voltage V_(th) is 0.5 V and the drain current is 1×10⁻⁹ Aat V_(gs) of 0.5 V, 1×10⁻¹³ A at V_(gs) of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of−0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of thetransistor is 1×10⁻¹⁹ A or lower at V_(gs) of −0.5 V or at V_(gs) in therange of −0.8 V to −0.5 V; therefore, it can be said that the off-statecurrent of the transistor is 1×10⁻¹⁹ A or lower. Since there is V_(gs)at which the drain current of the transistor is 1×10⁻²² A or lower, itmay be said that the off-state current of the transistor is 1×10⁻²² A orlower.

In this specification and the like, the off-state current of atransistor with a channel width W is sometimes represented by a currentvalue in relation to the channel width W or by a current value per givenchannel width (e.g., 1 μm). In the latter case, the off-state currentmay be expressed in the unit with the dimension of current per length(e.g., A/μm).

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be an off-state current at room temperature, 60° C.,85° C., 95° C., or 125° C. Alternatively, the off-state current may bean off-state current at a temperature at which the required reliabilityof a semiconductor device or the like including the transistor isensured or a temperature at which the semiconductor device or the likeincluding the transistor is used (e.g., temperature in the range of 5°C. to 35° C.). The description “an off-state current of a transistor islower than or equal to I” may refer to a situation where there is V_(gs)at which the off-state current of a transistor is lower than or equal toI at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature atwhich the required reliability of a semiconductor device or the likeincluding the transistor is ensured, or a temperature at which thesemiconductor device or the like including the transistor is used (e.g.,temperature in the range of 5° C. to 35° C.).

The off-state current of a transistor depends on voltage V_(ds) betweenits drain and source in some cases. Unless otherwise specified, theoff-state current in this specification may be an off-state current atV_(ds) of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12V, 16 V, or 20 V. Alternatively, the off-state current may be anoff-state current at V_(ds) at which the required reliability of asemiconductor device or the like including the transistor is ensured orV_(ds) at which the semiconductor device or the like including thetransistor is used. The description “an off-state current of atransistor is lower than or equal to I” may refer to a situation wherethere is V_(gs) at which the off-state current of a transistor is lowerthan or equal to I at V_(ds) of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V,3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, V_(ds) at which the requiredreliability of a semiconductor device or the like including thetransistor is ensured, or V_(ds) at which the semiconductor device orthe like including the transistor is used.

In the above description of off-state current, a drain may be replacedwith a source. That is, the off-state current sometimes refers tocurrent that flows through a source of a transistor in an off state.

In this specification and the like, the term “leakage current” sometimesexpresses the same meaning as off-state current. In this specificationand the like, the off-state current sometimes refers to current thatflows between a source and a drain when a transistor is off, forexample.

In this specification and the like, the threshold voltage of atransistor refers to a gate voltage (V_(g)) at which a channel is formedin the transistor. Specifically, in a graph where the horizontal axisrepresents the gate voltage (V_(g)) and the vertical axis represents thesquare root of drain current (I_(d)), the threshold voltage of atransistor may refer to a gate voltage (V_(g)) at the intersection ofthe square root of drain current (I_(d)) of 0 (I_(d)=0 A) and anextrapolated straight line that is tangent with the highest inclinationto a plotted curve (V_(g)−√I_(d) characteristics). Alternatively, thethreshold voltage of a transistor may refer to a gate voltage (V_(g)) atwhich the value of I_(d)[A]×L [μm]/W [μm] is 1×10⁻⁹ [A] where L ischannel length and W is channel width.

In this specification and the like, a “semiconductor” can havecharacteristics of an “insulator” when the conductivity is sufficientlylow, for example. Further, a “semiconductor” and an “insulator” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “insulator” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled an “insulator” in some cases. Similarly, an “insulator” in thisspecification and the like can be called a “semiconductor” in somecases. An “insulator” in this specification and the like can be called a“semi-insulator” in some cases.

In this specification and the like, a “semiconductor” can havecharacteristics of a “conductor” when the conductivity is sufficientlyhigh, for example. Further, a “semiconductor” and a “conductor” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “conductor” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled a “conductor” in some cases. Similarly, a “conductor” in thisspecification and the like can be called a “semiconductor” in somecases.

In this specification and the like, an impurity in a semiconductorrefers to an element that is not a main component of the semiconductor.For example, an element with a concentration of lower than 0.1 atomic %is an impurity. If a semiconductor contains an impurity, the density ofstates (DOS) may be formed therein, the carrier mobility may bedecreased, or the crystallinity may be decreased, for example. In thecase where the semiconductor includes an oxide semiconductor, examplesof the impurity which changes the characteristics of the semiconductorinclude Group 1 elements, Group 2 elements, Group 13 elements, Group 14elements, Group 15 elements, and transition metals other than the maincomponents; specific examples include hydrogen (also included in water),lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. Whenthe semiconductor is an oxide semiconductor, oxygen vacancies may beformed by entry of impurities such as hydrogen, for example.Furthermore, in the case where the semiconductor includes silicon,examples of the impurity which changes the characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described with reference to FIG. 1 to FIG. 35C.

One embodiment of the present invention is a semiconductor deviceincluding a transistor. The transistor includes a first gate electrode,a first insulating film over the first gate electrode, an oxidesemiconductor film over the first insulating film, a second insulatingfilm over the oxide semiconductor film, a second gate electrode over thesecond insulating film, and a third insulating film over the oxidesemiconductor film and the second gate electrode. The oxidesemiconductor film includes a channel region overlapping with the gateelectrode, a source region in contact with the third insulating film,and a drain region in contact with the third insulating film. The firstgate electrode and the second gate electrode are electrically connectedto each other.

The transistor includes a first region where the maximum value of thefield-effect mobility of the transistor at a gate voltage of higher than0 V and lower than or equal to 10 V is larger than or equal to 40 cm²/Vsand smaller than 150 cm²/Vs, a second region where the threshold voltageis higher than or equal to −1 V and lower than or equal to 1 V, a thirdregion where the S value is smaller than 0.3 V/decade, and a fourthregion where the off-state current is lower than 1×10⁻¹² A/cm², andμ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to 1 and smallerthan 1.5 where μ_(FE)(max) represents the maximum value of thefield-effect mobility of the transistor and μ_(FE)(V_(g)=2V) representsthe value of the field-effect mobility of the transistor at a gatevoltage of 2 V.

In some cases, μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to1.5 and smaller than 3 in the transistor.

The above structure can also be described as follows: a semiconductordevice of one embodiment of the present invention is a transistor inwhich an oxide semiconductor film is included in a channel region, andthe field-effect mobility, the threshold voltage, the off-state current,and the S value of the transistor are excellent. Such a semiconductordevice can be suitably used as a transistor in a pixel of an organic ELdisplay or a transistor in a driver circuit of an organic EL display.

In some cases, the transistor includes a first region where the maximumvalue of the field-effect mobility of the transistor at a gate voltageof higher than 0 V and lower than or equal to 10 V is larger than orequal to 10 cm²/Vs and smaller than 100 cm²/Vs, a second region wherethe threshold voltage is higher than or equal to −1 V and lower than orequal to 1 V, a third region where the S value is smaller than 0.3V/decade, and a fourth region where the off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 3 and smaller than 10 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents the value of the field-effect mobility of the transistor at agate voltage of 2 V.

The structure is also described as follows: a semiconductor device ofone embodiment of the present invention is a transistor in which anoxide semiconductor film is included in a channel region, and thetransistor is highly reliable because of its high heat resistance andstable physical properties. Such a semiconductor device can be suitablyused as a power device. For example, such a semiconductor device can besuitably used as a semiconductor device in an electric power convertersuch as an inverter or a converter. As other examples, such asemiconductor device can be used for inverter control in an electricvehicle, a hybrid vehicle, an air conditioner, or the like, variousgeneral-purpose motors, or the like. In this embodiment, an oxidesemiconductor which is one embodiment of the present invention isdescribed.

<1-1. Oxide Semiconductor Film>

First, an oxide semiconductor film that can be used in a transistor ofone embodiment of the present invention is described with reference toFIG. 4A to FIG. 13.

Indium is preferably contained in an oxide semiconductor film. Inparticular, indium and zinc are preferably contained. In addition,aluminum, gallium, yttrium, tin, or the like is preferably contained.Furthermore, one or more elements selected from boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the likemay be contained.

Here, the case where an oxide semiconductor film contains indium, anelement M, and zinc is considered. The element M is aluminum, gallium,yttrium, tin, or the like. Other elements that can be used as theelement M include boron, silicon, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, and magnesium. Note that two or more of the above elements maybe used in combination as the element M. The terms of the atomic ratioof indium, the element M, and zinc contained in the oxide semiconductorfilm are denoted by [In], [M], and [Zn], respectively.

<1-2. Structure of Oxide Semiconductor Film>

FIGS. 4A and 4B are schematic views of an oxide semiconductor film inone embodiment of the present invention.

FIG. 4A is a schematic view of a top surface of an oxide semiconductorfilm (a-b plane direction), and FIG. 4B is a schematic view of a crosssection of the oxide semiconductor film (c-axis direction) formed over asubstrate Sub.

FIGS. 4A and 4B illustrate an example in which the oxide semiconductorfilm is formed over the substrate; however, one embodiment of thepresent invention is not limited to this example and an insulating filmsuch as a base film or an interlayer film or another semiconductor filmsuch as an oxide semiconductor film may be formed between the substrateand the oxide semiconductor film.

The oxide semiconductor film of one embodiment of the present inventionis a composite oxide semiconductor having a structure in which Region A1and Region B1 are mixed as shown in FIGS. 4A and 4B. Therefore, in thefollowing description, the oxide semiconductor film is referred to as acomposite oxide semiconductor in some cases.

Region A1 shown in FIGS. 4A and 4B is high in In with[In]:[M]:[Zn]=x:y:z (x>0, y≥0, z≥0). In contrast, Region B1 is low in Inwith [In]:[M]:[Zn]=a:b:c (a>0, b>0, c>0).

Note that in this specification, for example, when the atomic ratio ofIn to the element M in Region A1 is greater than the atomic ratio of Into the element M in Region B1, Region A1 has higher In concentrationthan Region B1. Therefore, in this specification, Region A1 is alsoreferred to as an In-rich region, and Region B1 is also referred to asan In-poor region.

For example, the In concentration in Region A1 is 1.1 or more times,preferably 2 to 10 times that in Region B1. Region A1 is an oxidecontaining at least In and does not necessarily contain the element Mand Zn.

The atomic ratio of elements included in the composite oxidesemiconductor of one embodiment of the present invention will bedescribed here.

A phase diagram in FIG. 8 can be used to show the atomic ratio ofelements in the case where Region A1 in the composite oxidesemiconductor contains In, the element M, and Zn. The atomic ratio of Into the element M and Zn is denoted by x:y:z. This atomic ratio can beshown as coordinates (x:y:z) in FIG. 8. Note that the proportion ofoxygen atoms is not illustrated in FIG. 8.

In FIG. 8, dashed lines correspond to a line representing the atomicratio of [In]:[M]:[Zn]=(1+α):(1−α): 1 (−1≤α≤1), a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):2, a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):3, a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):4, and a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):5.

Dashed-dotted lines correspond to a line representing the atomic ratioof [In]:[M]:[Zn]=1:1:β (β≥0), a line representing the atomic ratio of[In]:[M]:[Zn]=1:2:3, a line representing the atomic ratio of[In]:[M]:[Zn]=1:3:3, a line representing the atomic ratio of[In]:[M]:[Zn]=1:4:3, a line representing the atomic ratio of[In]:[M]:[Zn]=1:7:3, a line representing the atomic ratio of[In]:[M]:[Zn]=2:1:3, and a line representing the atomic ratio of[In]:[M]:[Zn]=5:1:β.

An oxide semiconductor having the atomic ratio of [In]:[M]:[Zn]=0:2:1 ora neighborhood thereof in FIG. 8 tends to have a spinel crystalstructure.

Region A2 in FIG. 8 represents an example of a preferred range of atomicratios of indium to the element M and zinc contained in Region A1. Notethat Region A2 includes atomic ratios on a line representing the atomicratio of [In]:[M]:[Zn]=(1+γ):0:(1−γ) (−1≤γ≤1).

Region B2 in FIG. 8 represents an example of a preferred range of atomicratios of indium to the element M and zinc contained in Region B1. Notethat Region B2 includes atomic ratios from [In]:[M]:[Zn]=4:2:3 to[In]:[M]:[Zn]=4:2:4.1 and a neighborhood thereof. The neighborhoodincludes an atomic ratio of [In]:[M]:[Zn]=5:3:4. Region B2 includes anatomic ratio of [In]:[M]:[Zn]=5:1:6 and a neighborhood thereof.

Region A2 with high In concentrations provides a higher conductivitythan Region B2 and has a function of increasing carrier mobility(field-effect mobility). Therefore, the on-state current and carriermobility of a transistor using an oxide semiconductor film includingRegion A1 can be increased.

In contrast, Region B2 with low In concentrations provides a lowerconductivity than Region A2 and has a function of decreasing leakagecurrent. Therefore, the off-state current of a transistor using an oxidesemiconductor film including Region B1 can be decreased.

In the oxide semiconductor film of one embodiment of the presentinvention, Region A1 and Region B1 form a composite. That is, carriermovement occurs easily in Region A1, whereas carrier movement does notoccur easily in Region B1. Therefore, an oxide semiconductor of oneembodiment of the present invention can be used as a material with highcarrier mobility, excellent switching characteristics, and favorablesemiconductor characteristics.

For example, a plurality of Regions A1 are present in particulate form(in cluster form) in the a-b plane direction and the c-axis direction asshown in FIG. 4A. Note that clusters may be distributed unevenly andirregularly. A plurality of clusters overlap each other or are connectedto each other in some cases. For example, in some cases, shapes eachincluding a cluster overlapping with another cluster are connected toeach other, so that Region A1 is observed to extend in a cloud-likemanner.

Note that the switching characteristics of the transistor are degraded(for example, the off-state current of the transistor is increased) whenall of Regions A1 are connected in the a-b plane direction; thus,Regions A1 are preferably scattered in Region B1 as shown in FIGS. 4Aand 4B. Therefore, Region A1 can exist in a state of beingthree-dimensionally surrounded with Region B1. That is, Region A1 isenclosed by Region B1.

Region B1 can also be regarded as having a structure including a cluster(also referred to as a second cluster) that is different from a clusterincluded in Region A1 (also referred to as a first cluster). In thestructure, Region B1 includes a plurality of second clusters andincludes a portion in which the second clusters are connected to eachother. In other words, the first cluster included in Region A1 includesa portion where the first cluster and another first cluster areconnected to each other in a cloud-like manner, and the second clusterincluded in Region B1 includes a portion where the second cluster andanother second cluster are connected to each other in a cloud-likemanner.

Note that the proportion of scattered Regions A1 can be adjusted bychanging, for example, the formation conditions or composition of thecomposite oxide semiconductor. For example, it is possible to form acomposite oxide semiconductor with a low proportion of Regions A1 or acomposite oxide semiconductor with a high proportion of Regions A1. Forexample, FIGS. 5A and 5B show a composite oxide semiconductor in whichthe proportion of Regions A1 is lower than the proportion of Regions A1in the composite oxide semiconductor shown in FIGS. 4A and 4B. FIG. 5Ais a schematic view corresponding to FIG. 4A, and FIG. 5B is a schematicview corresponding to FIG. 4B. In a composite oxide semiconductor of oneembodiment of the present invention, the proportion of Regions A1 is notalways lower than that of Region B1. In a composite oxide semiconductorwith an extremely high proportion of Regions A1, depending on theobservation range, Region B1 is sometimes formed in Region A1. The sizeof the particulate region of Region A1 can be adjusted appropriately bychanging, for example, the formation conditions or composition of thecomposite oxide semiconductor.

In some cases, the boundary between Region A1 and Region B1 is notclearly observed. The sizes of Region A1 and Region B1 can be measuredwith energy dispersive X-ray spectroscopy (EDX) mapping images obtainedby EDX. For example, the diameter of a cluster in Region A1 is greaterthan or equal to 0.1 nm and less than or equal to 2.5 nm in the EDXmapping image of a cross-sectional photograph or a plan-view photographin some cases. Note that the diameter of the cluster is preferablygreater than or equal to 0.5 nm and less than or equal to 1.5 nm.

As described above, an oxide semiconductor of one embodiment of thepresent invention is a composite oxide semiconductor in which Region A1and Region B1 are mixed and have different functions that arecomplementary. For example, when an oxide semiconductor of oneembodiment of the present invention is an In—Ga—Zn oxide (hereinafterreferred to as IGZO), in which Ga is used as the element M, the oxidesemiconductor of one embodiment of the present invention can be calledcomplementary IGZO (abbreviation: C/IGZO).

In contrast, when Region A1 and Region B1 are stacked in a layeredmanner, for example, interaction does not take place or is unlikely totake place between Region A1 and Region B1, so that the function ofRegion A1 and that of Region B1 are independently performed in somecases. In that case, even when the carrier mobility is increased owingto Regions A1, the off-state current of the transistor might beincreased. Therefore, in the case where an oxide semiconductor of oneembodiment of the present invention is the above-described compositeoxide semiconductor or C/IGZO, a function of achieving high carriermobility and a function of achieving excellent switching characteristicscan be obtained at the same time. This is an advantageous effectobtained by using a composite oxide semiconductor of one embodiment ofthe present invention.

Note that in the case where the oxide semiconductor is deposited with asputtering apparatus, a film having an atomic ratio deviated from theatomic ratio of the target is formed. Especially for zinc, [Zn] in theatomic ratio of a deposited film is smaller than that in the atomicratio of the target in some cases depending on the substrate temperatureduring deposition.

Note that characteristics of the composite oxide semiconductor of oneembodiment of the present invention are not uniquely determined by theatomic ratio. Therefore, the illustrated regions represent preferredatomic ratios of Region A1 and Region B1 of the composite oxidesemiconductor; a boundary therebetween is not clear.

An oxide semiconductor according to the present invention is not limitedto the above description. FIGS. 6A and 6B and FIGS. 7A and 7B areschematic views of oxide semiconductor films having structures differentfrom the structure of the above-described oxide semiconductor film. FIG.6A and FIG. 7A are the schematic views of top surfaces of compositeoxide semiconductors (a-b plane direction). FIG. 6B and FIG. 7B are theschematic views of cross sections of composite oxide semiconductors(c-axis direction) each formed over a substrate Sub. Note that for thestructures of the oxide semiconductor films shown in FIGS. 6A and 6B andFIGS. 7A and 7B, the description of the structure of the oxidesemiconductor film shown in FIGS. 4A and 4B can be referred to exceptthe point to be described later.

Oxide semiconductors are classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in thea-b plane direction, and its crystal structure has distortion. Note thatthe distortion in the CAAC-OS is a portion where the direction of alattice arrangement changes between a region with a regular latticearrangement and another region with a regular lattice arrangement.

In FIG. 6A and FIG. 7A, a plurality of nanocrystals are schematicallyshown by dashed lines. The shape of the nanocrystal is basicallyhexagon. However, the shape is not always a regular hexagon and is anon-regular hexagon in some cases. At the distortion, a polygonalnanocrystal such as a pentagonal nanocrystal or a heptagonal nanocrystalis included in some cases.

Note that a clear grain boundary cannot be observed even in the vicinityof distortion in the CAAC-OS. That is, a lattice arrangement isdistorted so that formation of a grain boundary is inhibited. This isprobably because the CAAC-OS can tolerate distortion owing to a lowdensity of arrangement of oxygen atoms in an a-b plane direction, theinteratomic bond distance changed by substitution of a metal element,and the like.

Furthermore, FIG. 6B and FIG. 7B schematically show that nanocrystalshave c-axis alignment and the c-axes are aligned in a directionsubstantially perpendicular to a surface over which the CAAC-OS film isformed (also referred to as a formation surface) or the top surface ofthe CAAC-OS film. The CAAC-OS has a layered crystal structure (alsoreferred to as a layered structure) having c-axis alignment and includesa layer containing indium and oxygen (hereinafter referred to as an Inlayer) and a layer containing the element M, zinc, and oxygen(hereinafter referred to as an (M,Zn) layer) that are stacked.

Note that indium and the element M are replaced with each other in somecases. Therefore, when part of the elements Min the (M,Zn) layer arereplaced with indium, the layer can also be referred to as an (In,M,Zn)layer. In that case, the In layer and the (In,M,Zn) layer are stacked inthe layered structure.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different nanocrystals in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor. The a-like OS includes a void or alow-density region. That is, the a-like OS has an unstable structure,compared to the nc-OS and the CAAC-OS.

Oxide semiconductors have various structures and various properties. Anoxide semiconductor of the present invention may be a composite oxidesemiconductor including two or more of the amorphous oxidesemiconductor, the polycrystalline oxide semiconductor, the a-like OS,the nc-OS, and the CAAC-OS. Region A1 and Region B1 may have differentcrystallinities.

For example, Region A1 is preferably a non-single-crystal. Note that inthe case where Region A1 has crystallinity, when Region A1 is formed ofindium, Region A1 tends to have a tetragonal crystal structure.Furthermore, when Region A1 is formed of indium oxide([In]:[M]:[Zn]=x:0:0 (x>0)), Region A1 tends to have a bixbyite crystalstructure. Furthermore, when Region A1 is formed of an In—Zn oxide([In]:[M]:[Zn]=x:0:z (x>0, z>0)), Region A1 tends to have a layeredcrystal structure.

Region B1 includes a CAAC-OS. Note that Region B1 does not necessarilyinclude only a CAAC-OS and may also include a region of apolycrystalline oxide semiconductor, an nc-OS, or the like.

The CAAC-OS is an oxide semiconductor with high crystallinity. Incontrast, in the CAAC-OS, a reduction in electron mobility due to thegrain boundary is less likely to occur because a clear grain boundarycannot be observed. Entry of impurities, formation of defects, or thelike might decrease the crystallinity of an oxide semiconductor. Thismeans that the CAAC-OS has small amounts of impurities and defects(e.g., oxygen vacancies). Thus, with the CAAC-OS, a composite oxidesemiconductor is physically stable; thus, a composite oxidesemiconductor which is resistant to heat and has high reliability can beprovided.

Note that the proportion of scattered Regions A1 can be adjusted bychanging, for example, the formation conditions or composition of thecomposite oxide semiconductor. For example, as shown in FIG. 7A and FIG.7B, it is possible to form a composite oxide semiconductor with a lowproportion of Regions A1 or a composite oxide semiconductor with a highproportion of Regions A1.

<1-3. Transistor Including Oxide Semiconductor Film>

Next, the case where the above-described oxide semiconductor film isused for a transistor will be described.

With the use of the composite oxide semiconductor in a transistor, thetransistor can have high carrier mobility and high switchingcharacteristics. In addition, the transistor can have high reliability.

An oxide semiconductor film with low carrier density is preferably usedfor the transistor. For example, an oxide semiconductor film whosecarrier density is lower than 8×10¹¹/cm³, preferably lower than1×10¹¹/cm³, or further preferably lower than 1×10¹⁰/cm³, and greaterthan or equal to 1×10⁻⁹/cm³ is used as the oxide semiconductor film.

In order to reduce the carrier density of the oxide semiconductor film,the impurity concentration in the oxide semiconductor film is reduced sothat the density of defect states can be reduced. In this specificationand the like, a state with a low impurity concentration and a lowdensity of defect states is referred to as a highly purified intrinsicor substantially highly purified intrinsic state. A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has few carrier generation sources, and thus can have a low carrierdensity. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has a low density of defect statesand accordingly has a low density of trap states in some cases.

Charges trapped by the trap states in the oxide semiconductor film takea long time to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorhaving a high density of trap states has unstable electricalcharacteristics in some cases.

To obtain stable electrical characteristics of the transistor, it iseffective to reduce the concentration of impurities in the oxidesemiconductor film. In order to reduce the concentration of impuritiesin the oxide semiconductor film, the concentration of impurities in afilm that is adjacent to the oxide semiconductor film is preferablyreduced. As examples of the impurities, hydrogen, nitrogen, alkalimetal, alkaline earth metal, iron, nickel, silicon, and the like aregiven.

Here, the influence of impurities in the oxide semiconductor film willbe described.

When silicon or carbon that is one of Group 14 elements is contained inthe oxide semiconductor film, defect states are formed. Thus, theconcentration of silicon or carbon in the oxide semiconductor and aroundan interface with the oxide semiconductor (measured by secondary ionmass spectrometry (SIMS)) is set lower than or equal to 2×10¹⁸atoms/cm³, and preferably lower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor film contains alkali metal or alkalineearth metal, defect states are formed and carriers are generated, insome cases. Thus, a transistor including an oxide semiconductor filmwhich contains alkali metal or alkaline earth metal is likely to benormally on. Therefore, it is preferable to reduce the concentration ofalkali metal or alkaline earth metal in the oxide semiconductor film.Specifically, the concentration of alkali metal or alkaline earth metalin the oxide semiconductor film measured by SIMS is set lower than orequal to 1×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁶atoms/cm³.

When the oxide semiconductor film contains nitrogen, the oxidesemiconductor film easily becomes n-type by generation of electronsserving as carriers and an increase of carrier density. Thus, atransistor whose semiconductor includes an oxide semiconductor thatcontains nitrogen is likely to be normally-on. For this reason, nitrogenin the oxide semiconductor is preferably reduced as much as possible;the nitrogen concentration measured by SIMS is set, for example, lowerthan 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³,and still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and thus causes an oxygen vacancy(V_(o)), in some cases. Due to entry of hydrogen into the oxygen vacancy(V_(o)), an electron serving as a carrier is generated in some cases.Furthermore, in some cases, bonding of part of hydrogen to oxygen bondedto a metal atom causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on. Accordingly, it is preferable thathydrogen in the oxide semiconductor be reduced as much as possible.Specifically, the hydrogen concentration measured by SIMS in the oxidesemiconductor is lower than 1×10²⁰ atoms/cm³, preferably lower than1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, andstill further preferably lower than 1×10¹⁸ atoms/cm³.

The oxygen vacancies (V_(o)) in the oxide semiconductor film can bereduced by introduction of oxygen into the oxide semiconductor film.That is, the oxygen vacancies (V_(o)) in the oxide semiconductor filmdisappear when the oxygen vacancies (V_(o)) are filled with oxygen.Accordingly, diffusion of oxygen in the oxide semiconductor film canreduce the oxygen vacancies (V_(o)) in a transistor and improve thereliability of the transistor.

As a method for introducing oxygen into the oxide semiconductor film,for example, an oxide in which oxygen content is higher than that in thestoichiometric composition is provided in contact with the oxidesemiconductor film. That is, in the oxide, a region including oxygen inexcess of that in the stoichiometric composition (hereinafter alsoreferred to as an excess oxygen region) is preferably formed. Inparticular, in the case of using an oxide semiconductor film in atransistor, an oxide including an excess oxygen region is provided in abase film, an interlayer film, or the like in the vicinity of thetransistor, whereby oxygen vacancies in the transistor are reduced, andthe reliability can be improved.

When an oxide semiconductor film with sufficiently reduced impurityconcentration is used for a channel formation region in a transistor,the transistor can have stable electrical characteristics.

<1-4. Method for Manufacturing Composite Oxide Semiconductor>

An example of a method for manufacturing the composite oxidesemiconductor shown in FIGS. 4A and 4B is described with reference toFIGS. 9A and 9B, FIG. 10, FIGS. 11A and 11B, FIGS. 12A and 12B, and FIG.13. A composite oxide semiconductor of one embodiment of the presentinvention can be formed with a sputtering apparatus.

<1-5. Sputtering Apparatus>

FIG. 9A is a cross-sectional view of a deposition chamber 2501 of thesputtering apparatus. FIG. 9B is a plan view of a magnet unit 2530 a anda magnet unit 2530 b of the sputtering apparatus.

The deposition chamber 2501 illustrated in FIG. 9A includes a targetholder 2520 a, a target holder 2520 b, a backing plate 2510 a, a backingplate 2510 b, a target 2500 a, a target 2500 b, a member 2542, and asubstrate holder 2570. Note that the target 2500 a is placed over thebacking plate 2510 a. The backing plate 2510 a is placed over the targetholder 2520 a. The magnet unit 2530 a is placed under the target 2500 awith the backing plate 2510 a therebetween. The target 2500 b is placedover the backing plate 2510 b. The backing plate 2510 b is placed overthe target holder 2520 b. The magnet unit 2530 b is placed under thetarget 2500 b with the backing plate 2510 b therebetween.

As illustrated in FIGS. 9A and 9B, the magnet unit 2530 a includes amagnet 2530N1, a magnet 2530N2, a magnet 2530S, and a magnet holder2532. The magnet 2530N1, the magnet 2530N2, and the magnet 2530S areplaced over the magnet holder 2532 in the magnet unit 2530 a. The magnet2530N1, the magnet 2530N2, and the magnet 2530S are spaced. Note thatthe magnet unit 2530 b has a structure similar to that of the magnetunit 2530 a. When the substrate 2560 is transferred into the depositionchamber 2501, the substrate 2560 is placed in contact with the substrateholder 2570.

The target 2500 a, the backing plate 2510 a, and the target holder 2520a are separated from the target 2500 b, the backing plate 2510 b, andthe target holder 2520 b by the member 2542. Note that the member 2542is preferably an insulator. The member 2542 may be a conductor or asemiconductor. The member 2542 may be a conductor or a semiconductorwhose surface is covered with an insulator.

The target holder 2520 a and the backing plate 2510 a are fixed to eachother with a screw (e.g., a bolt) and have the same potential. Thetarget holder 2520 a has a function of supporting the target 2500 a withthe backing plate 2510 a positioned therebetween. The target holder 2520b and the backing plate 2510 b are fixed to each other with a screw(e.g., a bolt) and have the same potential. The target holder 2520 b hasa function of supporting the target 2500 b with the backing plate 2510 bpositioned therebetween.

The backing plate 2510 a has a function of fixing the target 2500 a. Thebacking plate 2510 b has a function of fixing the target 2500 b.

Magnetic lines of force 2580 a and 2580 b formed by the magnet unit 2530a are illustrated in FIG. 9A.

As illustrated in FIG. 9B, the magnet unit 2530 a has a structure inwhich the magnet 2530N1 having a rectangular or substantiallyrectangular shape, the magnet 2530N2 having a rectangular orsubstantially rectangular shape, and the magnet 2530S having arectangular or substantially rectangular shape are fixed to the magnetholder 2532. The magnet unit 2530 a can be oscillated horizontally asshown by an arrow in FIG. 9B. For example, the magnet unit 2530 a may beoscillated with a beat of greater than or equal to 0.1 Hz and less thanor equal to 1 kHz.

The magnetic field over the target 2500 a changes in accordance withoscillation of the magnet unit 2530 a. The region with an intensemagnetic field is a high-density plasma region; thus, sputtering of thetarget 2500 a easily occurs in the vicinity of the region. The sameapplies to the magnet unit 2530 b.

<1-6. Manufacturing Flow of Composite Oxide Semiconductor>

FIG. 10 is a process flow chart showing a method for manufacturing acomposite oxide semiconductor.

The composite oxide semiconductor shown in FIGS. 4A and 4B ismanufactured through at least first to fourth processes shown in FIG.10.

[First Process: Process of Placing Substrate in Deposition Chamber]

The first process includes a step of placing a substrate in a depositionchamber (see Step S101 in FIG. 10).

In the first process, for example, the substrate 2560 is placed on thesubstrate holder 2570 of the deposition chamber 2501 shown in FIGS. 9Aand 9B.

The temperature of the substrate 2560 can be set higher than or equal toroom temperature (25° C.) and lower than or equal to 200° C., preferablyhigher than or equal to room temperature and lower than or equal to 130°C. The substrate temperature in the above range is suitable for the caseof using a large glass substrate. In particular, when the substratetemperature in deposition of a composite oxide semiconductor is roomtemperature, i.e., the substrate is not heated intentionally, thesubstrate can be favorably prevented from bending or warping.

The substrate 2560 may be cooled with a cooling mechanism or the likeprovided for the substrate holder 2570.

In the case where the temperature of the substrate 2560 is set higherthan or equal to 100° C. and lower than or equal to 130° C., water inthe composite oxide semiconductor can be removed. Water as an impurityis removed in this manner, whereby the composite oxide semiconductorshown in FIGS. 5A and 5B can be formed easily. Thus, the field-effectmobility and the reliability can be improved at the same time.

Furthermore, in the case where the temperature of the substrate 2560 isset higher than or equal to 100° C. and lower than or equal to 130° C.,the sputtering apparatus can be prevented from warping due to overheat.Accordingly, semiconductor devices can be manufactured with higherproductivity. The productivity is thus stabilized, so that a large-scaleproduction apparatus is easy to employ. Thus, a large display deviceincluding a large substrate can be easily manufactured.

[Second Process: Process of Introducing Gas into Deposition Chamber]

The second process includes a step of introducing a gas into thedeposition chamber (see Step S201 in FIG. 10).

In the second process, for example, a gas is introduced into thedeposition chamber 2501 in FIGS. 9A and 9B. One or both of an argon gasand an oxygen gas are introduced as the gas. Note that instead of anargon gas, an inert gas such as helium, xenon, or krypton can be used.

The proportion of oxygen in the whole deposition gas in forming acomposite oxide semiconductor using an oxygen gas is referred to as anoxygen flow rate percentage in some cases. The oxygen flow ratepercentage in forming a composite oxide semiconductor is higher than orequal to 0% and lower than or equal to 30%, preferably higher than orequal to 5% and lower than or equal to 30%, further preferably higherthan or equal to 7% and lower than or equal to 15%.

In the case of depositing the composite oxide semiconductor shown inFIGS. 5A and 5B, the oxygen flow rate percentage in deposition at roomtemperature is set higher than 30% and lower than 70%, preferably higherthan 30% and lower than or equal to 50%. Furthermore, the oxygen flowrate percentage in thermal deposition (e.g., at a temperature higherthan or equal to 70° C. and lower than or equal to 150° C.) is sethigher than or equal to 10% and lower than or equal to 50%, preferablyhigher than or equal to 30% and lower than or equal to 50%.

In the case of forming the composite oxide semiconductor shown in FIGS.6A and 6B or the composite oxide semiconductor shown in FIGS. 7A and 7B,a mixed gas of a rare gas and oxygen is used, and the proportion ofoxygen to the rare gas is set higher than or equal to 70% and lower thanor equal to 100%.

In addition, increasing the purity of the gas is necessary. For example,as an oxygen gas or an argon gas used as the gas, a gas which is highlypurified to have a dew point of −40° C. or lower, preferably −80° C. orlower, further preferably −100° C. or lower, still further preferably−120° C. or lower is used, whereby entry of moisture or the like intothe composite oxide semiconductor can be minimized.

The deposition chamber 2501 is preferably evacuated to high vacuum(about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an entrapment vacuum evacuation pumpsuch as a cryopump so that water or the like, which is an impurity forthe composite oxide semiconductor, is removed as much as possible. Inparticular, the partial pressure of gas molecules corresponding to H₂O(gas molecules corresponding to m/z=18) in the deposition chamber 2501in the standby mode of the sputtering apparatus is preferably lower thanor equal to 1×10⁻⁴ Pa, further preferably lower than or equal to 5×10⁻⁵Pa.

[Third Process: Process of Applying Voltage to Target]

The third process includes a step of applying voltage to a target (seeStep S301 in FIG. 10).

In the third process, for example, voltage is applied to the targetholder 2520 a and the target holder 2520 b in FIGS. 9A and 9B. As anexample, a potential applied to a terminal V1 connected to the targetholder 2520 a is lower than a potential applied to a terminal V2connected to the substrate holder 2570. A potential applied to aterminal V4 connected to the target holder 2520 b is lower than thepotential applied to the terminal V2 connected to the substrate holder2570. The potential applied to the terminal V2 connected to thesubstrate holder 2570 is a ground potential. A potential applied to aterminal V3 connected to the magnet holder 2532 is a ground potential.

Note that the potentials applied to the terminals V1, V2, V3, and V4 arenot limited to the above potentials. Not all the target holder 2520, thesubstrate holder 2570, and the magnet holder 2532 are necessarilysupplied with potentials. For example, the substrate holder 2570 may beelectrically floating. Note that it is assumed that a power sourcecapable of controlling a potential applied to the terminal V1 iselectrically connected to the terminal V1. As the power source, a DCpower source, an AC power source, or an RF power source may be used.

As the target 2500 a and the target 2500 b, a target including indium,the element M (M is Al, Ga, Y, or Sn), zinc, and oxygen is preferablyused. For example, an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1[atomic ratio]) or an In—Ga—Zn metal oxide target (In:Ga:Zn=5:1:7[atomic ratio]) can be used as the target 2500 a and the target 2500 b.In the following description, the case of using an In—Ga—Zn metal oxidetarget (In:Ga:Zn=4:2:4.1 [atomic ratio]) is described.

Note that by using a sputtering target including a polycrystalline oxidehaving a plurality of crystal grains as the target 2500 a and the target2500 b, a composite oxide semiconductor that has crystallinity and isshown in FIGS. 4A and 4B or FIG. 8 is obtained easily.

[Fourth Process: Process of Depositing Composite Oxide Semiconductor onSubstrate]

The fourth process includes a step in which sputtered particles areejected from the target and a composite oxide semiconductor is depositedon the substrate (see Step S401 in FIG. 10).

In the fourth process, for example, in the deposition chamber 2501illustrated in FIGS. 9A and 9B, an argon gas or an oxygen gas is ionizedto be separated into cations and electrons, and plasma is created. Then,the cations in the plasma are accelerated toward the targets 2500 a and2500 b by the potentials applied to the target holders 2520 a and 2520b. Sputtered particles are generated when the cations collide with theIn—Ga—Zn metal oxide target, and the sputtered particles are depositedon the substrate 2560.

Note that in an In—Ga—Zn metal oxide target with an atomic ratio ofIn:Ga:Zn=4:2:4.1 or 5:1:7 that is used as the targets 2500 a and 2500 b,a plurality of crystal grains with different compositions are includedin some cases. In most cases, for example, the diameters of theplurality of crystal grains are each 10 μm or less. In the case where,for example, crystal grains with a high proportion of In are included inthe In—Ga—Zn metal oxide target, the proportion of Region A1 describedabove is increased in some cases.

<1-7. Deposition Model>

In the fourth process, a deposition model shown in FIGS. 11A and 11B canbe presumed.

FIGS. 11A and 11B are cross-sectional views of the vicinity of thetarget 2500 a shown in FIGS. 9A and 9B. Note that in FIGS. 11A and 11B,the backing plate 2510 a, the target 2500 a, plasma 2190, cations 2192,sputtered particles 2194, and the like are shown.

[First Step]

In FIG. 11A, an argon gas or an oxygen gas is ionized and separated intothe cations 2192 and electrons (not illustrated), and the plasma 2190 isgenerated. After that, the cations 2192 in the plasma 2190 areaccelerated toward the target 2500 a (here, an In—Ga—Zn metal oxidetarget). The cations 2192 collide with the In—Ga—Zn metal oxide target,whereby the sputtered particles 2194 are generated and ejected from theIn—Ga—Zn metal oxide target. Note that from the In—Ga—Zn metal oxidetarget, first, Ga and Zn are preferentially sputtered as the sputteredparticles 2194.

Specifically, the cations 2192 collide with the In—Ga—Zn metal oxidetarget, so that Ga and Zn, which have lower relative atomic masses thanIn, are preferentially ejected from the In—Ga—Zn metal oxide target. Theejected In, Ga, and Zn are bonded to oxygen and then deposited on thesubstrate, whereby Region B1 shown in FIGS. 4A and 4B is formed.

Note that as shown in FIG. 11A, preferential sputtering of Ga and Zn asthe sputtered particles 2194 brings about a state in which In issegregated on a surface of the target 2500 a (here, a surface of theIn—Ga—Zn metal oxide target) in some cases. Note that in FIG. 11A,segregated In is shown as a cluster 2196.

[Second Step]

Then, as shown in FIG. 11B, the cluster 2196 (here, a cluster includingIn) is sputtered from the In—Ga—Zn metal oxide target after In issegregated, i.e., after the cluster 2196 is formed.

Specifically, In segregated at the surface of the In—Ga—Zn metal oxidetarget is ejected from the In—Ga—Zn metal oxide target as a structurelike a plurality of clusters. The segregated In having a structure likea plurality of clusters is bonded to oxygen and collides with Region B1deposited earlier, whereby Region A1 is deposited in cluster form (inparticulate form). Note that since the segregated In is ejected, In, Ga,and Zn exist at the surface of the In—Ga—Zn metal oxide target in astate closer to the original atomic ratio.

Note that as shown in FIG. 11B, in one region of the surface of thetarget 2500 a, In is segregated, and in another region of the surface ofthe target 2500 a, segregated In is ejected. That is, a mechanism of Insegregation and a mechanism of ejecting of segregated In work at thesame time, leading to a structure in which Region A1 is surrounded withRegion B1 and distributed unevenly and irregularly.

The deposition model including the first step and the second step isrepeated, whereby the composite oxide semiconductor that is oneembodiment of the present invention and shown in FIGS. 4A and 4B can beobtained.

Note that the formation method is not limited to a sputtering method; apulsed laser deposition (PLD) method, a plasma-enhanced chemical vapordeposition (PECVD) method, a thermal chemical vapor deposition (CVD)method, an atomic layer deposition (ALD) method, a vacuum evaporationmethod, or the like may be used. As an example of a thermal CVD method,a metal organic chemical vapor deposition (MOCVD) method can be given.

<1-8. Examination of Deposition Model>

Sample Z1 described below was formed to examine the above-describeddeposition model.

[Sample Z1]

In Sample Z1, an insulating film 82, an insulating film 84, an oxidesemiconductor film 88, and an insulating film 86 were sequentiallyformed over a glass substrate.

The insulating films 82 and 84 function as a base film. As theinsulating film 82, a 400-nm-thick silicon nitride film was formed witha PECVD apparatus. As the insulating film 84, a 50-nm-thick siliconoxynitride film was formed with a PECVD apparatus.

As the oxide semiconductor film 88, a 40-nm-thick In—Ga—Zn oxide filmwas formed with a sputtering apparatus. Note that the oxidesemiconductor film 88 was formed under the following conditions: thesubstrate temperature was 170° C., an argon gas at a flow rate of 35sccm and an oxygen gas at a flow rate of 15 sccm were introduced into achamber, the pressure was 0.2 Pa, and an AC power of 1500 W was suppliedto a metal oxide target (In:Ga:Zn=4:2:4.1 [atomic ratio]) placed in thesputtering apparatus.

Note that in this embodiment, the oxide semiconductor film 88 is assumedto be a sputtering target for forming a composite oxide semiconductor.

After the oxide semiconductor film 88 was formed, plasma treatment wasperformed on a surface of the oxide semiconductor film 88 using an argongas. This is assumed to be sputtering in a sputtering apparatus. Theplasma treatment was performed under the following conditions: an argongas at a flow rate of 100 sccm was introduced into a chamber; thepressure in the chamber was set to 40 Pa; and power of 1000 W wassupplied to an RF power source (27.12 MHz).

After the plasma treatment, the insulating film 86 was formed over theoxide semiconductor film 88. The insulating film 86 functions as aprotective insulating film. As the insulating film 86, a 100-nm-thicksilicon oxynitride film was formed with a PECVD apparatus.

Through the process, Sample Z1 for examining the deposition model wasformed.

Next, a high-angle annular dark field STEM (HAADF-STEM) image of a crosssection of Sample Z1 was observed. Note that for the HAADF-STEMobservation, JEM-ARM200F manufactured by JEOL Ltd. was used under theconditions where the acceleration voltage was 200 kV.

FIG. 13 shows the HAADF-STEM observations of Sample Z1.

As shown in FIG. 13, a structure 90 was formed over the oxidesemiconductor film 88. Furthermore, from the HAADF-STEM observations,the thickness of the oxide semiconductor film 88 was approximately 36 nmand the thickness of the structure 90 was approximately 11 nm.

To evaluate the composition of the structure 90, elementary analysis wasconducted at a point 1 shown in the oxide semiconductor film 88 and apoint 2 shown in the structure 90. Note that the elementary analysis wasconducted with an EDX apparatus JET-2300T. The beam diameter of theelementary analysis was set to 0.1 nmϕ.

Table 1 shows the EDX analysis results.

TABLE 1 In Ga Zn O Total point1 25.6 12.8 15.6 46.0 100 point2 66.0 4.81.2 27.9 100 Unit: atomic %

When the values for the point 1 shown in Table 1 are normalized by avalue of In of the metal oxide target, the atomic ratio isIn:Ga:Zn:O=4:2:2.4:7.2. Although the proportion of Zn atoms is slightlyapart from the proportion of Zn atoms in the composition of the metaloxide target, the composition of the oxide semiconductor film 88 roughlycorresponds to the composition of the metal oxide target. Meanwhile, thestructure 90 has a high proportion of In as shown in Table 1. Therefore,the structure 90 can also be regarded as a precipitate of In or aprecipitate of an indium oxide.

The precipitate of In or the precipitate of an indium oxide can beassumed as In segregated at the surface of the In—Ga—Zn metal oxidetarget in the above-described deposition model (i.e., the cluster 2196).This indicates that the above deposition model is quite appropriate.

Note that in the case of depositing the composite oxide semiconductorthat is one embodiment of the present invention and shown in FIGS. 6Aand 6B, a deposition model shown in FIGS. 12A and 12B can be presumed inthe fourth process.

FIGS. 12A and 12B are cross-sectional views of the vicinity of thetarget 2500 a shown in FIGS. 9A and 9B. Note that in FIGS. 12A and 12B,the backing plate 2510 a, the target 2500 a, the plasma 2190, thecations 2192, the sputtered particles 2194, and the like are shown.

In the deposition chamber 2501 illustrated in FIG. 12A, an argon gas oran oxygen gas is ionized and separated into the cations 2192 andelectrons (not illustrated), and the plasma 2190 is generated. Afterthat, the cations 2192 in the plasma 2190 are accelerated toward thetarget 2500 a (here, an In—Ga—Zn metal oxide target). The cations 2192collide with the In—Ga—Zn metal oxide target, whereby the sputteredparticles 2194 are generated and ejected from the In—Ga—Zn metal oxidetarget.

Here, the targets 2500 a and 2500 b have a polycrystalline structureincluding a plurality of crystal grains. In most cases, the diameters ofthe plurality of crystal grains are each 10 μm or less. Furthermore, insome cases, the plurality of crystal grains have different compositionsin the In—Ga—Zn metal oxide target with an atomic ratio ofIn:Ga:Zn=4:2:4.1 or 5:1:7, for example.

For example, in FIG. 12A, the target 2500 a includes at least a region2502 a in which the proportion of In atoms is lower than the proportionof In atoms in the target and a region 2504 a in which the proportion ofIn atoms is higher than the proportion of In atoms in the target.

First, the region 2502 a with a low proportion of In in the target 2500a is described.

As shown in FIG. 12A, the cations 2192 generated in the high-densityplasma region are accelerated toward the target 2500 a side by anelectric field, and then collide with the region 2502 a included in thetarget 2500 a. At this time, a cluster 2198 that is a flat-plate-likenanocrystal is separated from the region 2502 a. In most cases, thecluster includes two M-Zn—O layers and an In—O layer therebetween. Notethat along with the separation of the cluster 2198, the sputteredparticles 2194 are also ejected from the target 2500 a.

In some cases, the cluster 2198 includes a triangular plane, e.g.,regular triangular plane. In some cases, the cluster 2198 includes ahexagonal plane, e.g., regular hexagonal plane. However, the shape of aplane of the cluster 2198 is not limited to a triangle or a hexagon. Forexample, the plane may have a shape formed by combining two or moretriangles. For example, a quadrangle (e.g., rhombus) may be formed bycombining two triangles (e.g., regular triangles).

The thickness of the cluster 2198 is determined depending on the kind ofthe deposition gas and the like. The thickness of the cluster 2198 is,for example, greater than or equal to 0.4 nm and less than or equal to 1nm, and preferably greater than or equal to 0.6 nm and less than orequal to 0.8 nm. In addition, for example, the width of the cluster 2198is greater than or equal to 1 nm and less than or equal to 3 nm,preferably greater than or equal to 1.2 nm and less than or equal to 2.5nm.

The surface of the sputtered particle 2194 may be negatively orpositively charged when the sputtered particle 2194 passes through theplasma 2190. For example, the sputtered particle 2194 sometimes receivesa negative charge from O₂ ⁻ that is present in the plasma 2190. As aresult, oxygen atoms on the surface of the sputtered particle 2194 maybe negatively charged. In addition, when passing through the plasma2190, the sputtered particle 2194 is sometimes combined with In, theelement M, Zn, oxygen, or the like in the plasma 2190 to grow. Thus, thesputtered particle 2194 has an atom or an aggregate of several atoms.

The cluster 2198 and the sputtered particles 2194 that have passedthrough the plasma 2190 reach a surface of the substrate. Since thecluster 2198 has a flat-plate-like shape, it is deposited with its flatplane facing the surface of the substrate. Note that some of thesputtered particles 2194 are discharged to the outside by a vacuum pumpor the like because the sputtered particle 2194 is small in mass.

The sputtered particles 2194 reach the surface of the substrate. Thesputtered particles 2194 are more likely to be bonded to a side surfaceof the cluster 2198 than to a top surface of the cluster 2198. Thesputtered particles 2194 are deposited preferentially on the sidesurface of the cluster 2198 so as to fill a region where the cluster2198 is not formed. Since available bonds of the sputtered particle 2194are activated, the sputtered particle 2194 is chemically bonded to thecluster 2198 to form lateral growth portions. In other words, thesputtered particle 2194 enters a region between one cluster and anothercluster.

The lateral growth portions further grow in a lateral direction so as tofill the region between one cluster and another cluster (the region isalso referred to as a lateral growth buffer region, LGBR). The lateraldirection indicates a direction perpendicular to the c-axis in acluster, for example.

A reaction is likely to occur in which a sputtered particle is attachedonto the lateral growth portion of the cluster, oxygen diffused passingthrough the LGBR is attached onto the sputtered particle, and anothersputtered particle is attached similarly. It is assumed that solid-phasegrowth in the lateral direction occurs by the repetition of thereaction. Such lateral growth of the clusters can also be referred to asself-assembly.

The lateral growth portions further grow laterally and collide with eachother. The adjacent clusters are connected to each other using a portionwhere the lateral growth portions collide with each other as aconnecting portion. In other words, the sputtered particles fill theregion between one of the clusters and the other cluster by forming thelateral growth portions on the side surfaces of the clusters and causinglateral growth. In this manner, the lateral growth portions are formeduntil a region where a cluster is not formed is filled. This mechanismis similar to a deposition mechanism of an atomic layer deposition (ALD)method.

Therefore, even when a plurality of formed clusters are oriented indifferent directions, Region B1 including a CAAC-OS is formed withoutforming a clear grain boundary because the sputtered particles growlaterally to fill a gap between one of the clusters and the othercluster.

Note that in the CAAC-OS, a layered crystal structure is stable over awide range of compositions, and the bonding strength and theequilibration distance between a metal atom and an oxygen atom depend onthe metal atom. Therefore, it is assumed that the crystal structure ofthe CAAC-OS is tolerant to distortion. That is, the clusters aresmoothly connected (anchored) by the sputtered particles, so that acrystal structure different from a single crystal and a polycrystal isformed in the connecting portion. In other words, a crystal structurehaving distortion is formed in the connecting portion between theadjacent clusters. Owing to this, for example, in the connectingportion, the shape of a hexagonal top surface of a crystal structure ischanged to a pentagonal or heptagonal shape in some cases.

Next, the region 2504 a with a high proportion of In in the target 2500a is described.

The cations 2192 generated in the high-density plasma region areaccelerated toward the target 2500 a side by an electric field, and thencollide with the region 2504 a included in the target 2500 a. In thecase of using the In—Ga—Zn metal oxide target, Ga and Zn arepreferentially sputtered as the sputtered particles 2194. That is, thecations 2192 collide with the In—Ga—Zn metal oxide target, so that Gaand Zn, which have lower relative atomic masses than In, arepreferentially ejected from the In—Ga—Zn metal oxide target. Asdescribed above, the sputtered particles 2194 ejected are deposited onthe substrate to fill the region between the clusters 2198 ejected fromthe region 2502 a and deposited on the substrate, whereby the region B1is formed.

Furthermore, as shown in FIG. 12A, after Ga and Zn are preferentiallysputtered as the sputtered particles 2194, a state where In issegregated at the surface of the region 2504 a with a high proportion ofIn is obtained. Note that in FIG. 12A, segregated In is shown as thecluster 2196. The diameter of the cluster 2196 is preferablyapproximately greater than or equal to 0.5 nm and less than or equal to1.5 nm.

As shown in FIG. 12B, after In is segregated, i.e., after the cluster2196 is formed, the cluster 2196 is sputtered from the region 2504 awith a high proportion of In.

Specifically, In segregated at the surface of the In—Ga—Zn metal oxidetarget is ejected from the In—Ga—Zn metal oxide target as a structurelike a plurality of particulate clusters. The segregated In having astructure like a plurality of clusters is bonded to oxygen and collideswith Region B1 deposited earlier, whereby Region A1 having particulateclusters is deposited. Note that since the segregated In is ejected, In,Ga, and Zn existing at the surface of the region 2504 a have an atomicratio closer to the original atomic ratio.

Note that as shown in FIGS. 12A and 12B, in one region of the surface ofthe region 2504 a with a high proportion of In in the target 2500 a, Inis segregated, and in another region of the surface, segregated In isejected. That is, a mechanism of In segregation and a mechanism ofejecting of segregated In work at the same time.

Accordingly, the region 2504 a with a high proportion of In tends toform Region A1 because the cluster 2196 is easily formed in the region2504 a. Meanwhile, the region 2502 a with a low proportion of In tendsto form Region B1 by deposition of the ejected clusters and thesputtered particles bonded to oxygen on the substrate.

Consequently, Region A1 and Region B1 extend in a cloud-like manner andare distributed unevenly and irregularly. In the above-described manner,the composite oxide semiconductor that is one embodiment of the presentinvention and shown in FIGS. 6A and 6B can be obtained.

<1-9. Classification of Oxide Semiconductor>

Next, classification of oxide semiconductors is described.

Oxide semiconductors are classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofthe non-single-crystal oxide semiconductor include a CAAC-OS, apolycrystalline oxide semiconductor, an nc-OS, an a-like OS, and anamorphous oxide semiconductor.

From another perspective, oxide semiconductors are classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of the crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and have no fixed atomicarrangement, to have a flexible bond angle, and to have a short-rangeorder but have no long-range order, for example.

In other words, a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS has physical properties similarto those of an amorphous oxide semiconductor.

[CAAC-OS]

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors and has a plurality of c-axisaligned crystal parts (also referred to as pellets).

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hasfew impurities and defects (e.g., oxygen vacancies).

Note that an impurity means an element other than the main components ofan oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

[nc-OS]

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OSis analyzed by an out-of-plane method, a peak indicating orientationdoes not appear. That is, a crystal of an nc-OS does not haveorientation.

The nc-OS is an oxide semiconductor that has higher regularity than anamorphous oxide semiconductor. Thus, the nc-OS has a lower density ofdefect states than the a-like OS and the amorphous oxide semiconductor.Note that there is no regularity of crystal orientation betweendifferent pellets in the nc-OS. Therefore, the nc-OS has a higherdensity of defect states than the CAAC-OS in some cases.

[A-Like OS]

An a-like OS has a structure between the structure of an nc-OS and thestructure of an amorphous oxide semiconductor.

The a-like OS contains a void or a low-density region. The a-like OS hasan unstable structure because it contains a void.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofthe nc-OS and the density of the CAAC-OS are each higher than or equalto 92.3% and lower than 100% of the density of the single crystal oxidesemiconductor having the same composition. It is difficult to deposit anoxide semiconductor having a density lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor whose atomic ratio ofIn to Ga to Zn is 1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor whose atomic ratio of In to Ga to Zn is1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³and lower than 5.9 g/cm³, for example. In the case of the oxidesemiconductor whose atomic ratio of In to Ga to Zn is 1:1:1, the densityof the nc-OS and the density of the CAAC-OS are each higher than orequal to 5.9 g/cm³ and lower than 6.3 g/cm³, for example.

In the case where an oxide semiconductor having a certain compositiondoes not exist in a single crystal state, single crystal oxidesemiconductors with different compositions are combined at an adequateratio, which makes it possible to calculate a density equivalent to thatof a single crystal oxide semiconductor with the desired composition.The density of a single crystal oxide semiconductor having the desiredcomposition may be calculated using a weighted average with respect tothe combination ratio of the single crystal oxide semiconductors withdifferent compositions. Note that it is preferable to use as few kindsof single crystal oxide semiconductors as possible to calculate thedensity.

As described above, oxide semiconductors have various structures andvarious properties. In the oxide semiconductor film of one embodiment ofthe present invention, two or more of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS may be mixed.

Note that Region A1 described above is preferably non-single-crystal.Region B1 is preferably non-single-crystal. Region A1 and Region B1 mayhave different crystallinities.

<1-10. Characteristics of Transistor>

Next, general characteristics of a transistor are described withreference to FIGS. 14A and 14B, FIG. 15, FIG. 16, and FIGS. 17A to 17C.

[I_(d)−V_(g) Characteristics of Transistors]

First, drain current-gate voltage characteristics (I_(d)−V_(g)characteristics) of a transistor are described. FIG. 14A illustrates anexample of I_(d)−V_(g) characteristics of the transistor. FIG. 14A showsthe case where polycrystalline silicon is used for an active layer ofthe transistor for easy understanding. In FIG. 14A, the vertical axisand the horizontal axis represent I_(d) and V_(g), respectively.

As illustrated in FIG. 14A, I_(d)−V_(g) characteristics are broadlydivided into three regions. A first region, a second region, and a thirdregion are referred to as an off region (OFF region), a subthresholdregion, and an on region (ON region), respectively. A gate voltage at aboundary between the subthreshold region and the on region is referredto as a threshold voltage (V_(th)).

To obtain favorable characteristics of the transistor, it is preferablethat the drain current in the off region (also referred to as off-statecurrent or I_(off)) be low and the drain current in the on region (alsoreferred to as on-state current or I_(on)) be high. As an index of theon-state current of the transistor, the field-effect mobility is oftenused. The details of the field-effect mobility are described later.

To drive the transistor at a low voltage, the slope of the I_(d)−V_(g)characteristics in the subthreshold region is preferably steep. An indexof the degree of change in the I_(d)−V_(g) characteristics in thesubthreshold region is referred to as subthreshold swing (SS) or an Svalue. The S value is represented by the following formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{{S\; S} = {\min\mspace{11mu}\left( \frac{\partial V_{g}}{\partial{\log_{10}\left( I_{d} \right)}} \right)}} & (1)\end{matrix}$

The S value is a minimum value of the amount of change in gate voltagewhich is needed for changing a drain current by an order of magnitude inthe subthreshold region. As the S value is smaller, switching operationbetween on and off states can be performed rapidly.

[I_(d)−V_(d) Characteristics of Transistor]

Next, drain current-drain voltage characteristics (I_(d)−V_(d)characteristics) of a transistor are described. FIG. 14B illustrates anexample of I_(d)−V_(d) characteristics of the transistor. In FIG. 14B,the vertical axis and the horizontal axis represent I_(d) and V_(d),respectively.

As illustrated in FIG. 14B, the on region is further divided into tworegions. A first region and a second region are referred to as a linearregion and a saturation region, respectively. In the linear region,drain current increases in a parabola shape in accordance with theincrease in drain voltage. On the other hand, in the saturation region,drain current does not greatly change even when drain voltage changes.According to a vacuum tube, the linear region and the saturation regionare referred to as a triode region and a pentode region in some cases.

The linear region indicates the state where V_(g) is higher than V_(d)(V_(d)<V_(g)) in some cases. The saturation region indicates the statewhere V_(d) is higher than V_(g) (V_(g)<V_(d)) in some cases. However,in practice, the threshold voltage of the transistor needs to beconsidered. Thus, the state where a value obtained by subtracting thethreshold voltage of the transistor from the gate voltage is higher thanthe drain voltage (V_(d)<V_(g)−V_(th)) is referred to as the linearregion in some cases. Similarly, the state where a value obtained bysubtracting the threshold voltage of the transistor from the gatevoltage is lower than the drain voltage (V_(g)−V_(th)<V_(d)) is referredto as the saturation region in some cases.

The I_(d)−V_(d) characteristics of the transistor with which current inthe saturation region is constant are expressed as “favorablesaturation” in some cases. The favorable saturation of the transistor isimportant particularly when the transistor is used in an organic ELdisplay. For example, a transistor with favorable saturation is used asa transistor of a pixel of an organic EL display, whereby a change inluminance of the pixel can be suppressed even when the drain voltage ischanged.

[Analysis Model of Drain Current]

Next, an analysis model of the drain current is described. As theanalysis model of the drain current, analytic formulae of drain currentbased on gradual channel approximation (GCA) is known. On the basis ofGCA, the drain current of the transistor is represented by the followingformula (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{I_{d} = \left\{ \begin{matrix}{\mu\frac{W}{L}{C_{ox}\left\lbrack {{\left( {{Vg} - V_{th}} \right)V_{d}} - {\frac{1}{2}V_{d}^{2}}} \right\rbrack}\mspace{14mu}\ldots\mspace{14mu}\left( {{V_{g} - V_{th}} > V_{d}} \right)} \\{\mu\frac{W}{2L}{C_{ox}\left( {{Vg} - V_{th}} \right)}^{2}\mspace{14mu}\ldots\mspace{14mu}\left( {{V_{g} - V_{th}} \leq V_{d}} \right)}\end{matrix} \right.} & (2)\end{matrix}$

In the formula (2), the upper formula is a formula for drain current ina linear region and the lower formula is a formula for drain current ina saturation region. In the formula (2), I_(d) represents the draincurrent, μ represents the mobility of the active layer, L represents thechannel length of the transistor, W represents the channel width of thetransistor, C_(OX) represents the gate capacitance, V_(g) represents thegate voltage, V_(d) represents the drain voltage, and V_(th) representsthe threshold voltage of the transistor.

[Field-Effect Mobility]

Next, field-effect mobility is described. As an index of current drivecapability of a transistor, the field-effect mobility is used. Asdescribed above, the on region of the transistor is divided into thelinear region and the saturation region. From the characteristics in theregions, the field-effect mobility of the transistor can be calculatedon the basis of the analytic formulae of the drain current based on GCA.The field-effect mobility in the linear region and the field-effectmobility in the saturation region are referred to as linear mobility andsaturation mobility, respectively, when they need to be distinguishedfrom each other. The linear mobility is represented by the followingformula (3) and the saturation mobility is represented by the followingformula (4).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{\mu_{FE}^{lin} = {\frac{L}{{WC}_{OX}}\frac{\partial I_{d}}{\partial V_{g}}\frac{1}{V_{d}}}} & (3) \\\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack & \; \\{\mu_{FE}^{sat} = {\frac{2L}{{WC}_{OX}}\left( \frac{\partial\sqrt{I_{d}}}{\partial V_{g}} \right)^{2}}} & (4)\end{matrix}$

In this specification and the like, curves calculated from the formula(3) and the formula (4) are referred to as mobility curves. FIG. 15shows mobility curves calculated from the analytic formulae of draincurrent based on GCA. In FIG. 15, the I_(d)−V_(g) characteristics atV_(d)=10 V and the mobility curves of the linear mobility and thesaturation mobility at the time when GCA is assumed to be effective areshown together.

In FIG. 15, the I_(d)−V_(g) characteristics are calculated from theanalytic formulae of drain current based on GCA. The shapes of themobility curves can be a lead to understanding the state of the insideof the transistor.

As an example, FIG. 16 shows the measured I_(d)−V_(g) characteristics ofa FET including a CAAC-OS. In FIG. 16, the I_(d)−V_(g) characteristicsand the mobility curves of the saturation mobility and the linearmobility of the FET are shown together. Note that an oxide semiconductor(IGZO) film with an atomic ratio of In:Ga:Zn=1:1:1 is used as asemiconductor layer of the FET. The mobility curves of the saturationmobility and the linear mobility are each obtained from the I_(d)−V_(g)characteristics curve at V_(d)=10 V.

As shown in FIG. 16, when the FET shape follows GCA, the curve of thesaturation mobility plateaus in the saturation region and goes downgradually in the linear region.

<1-11. Fabrication of Transistor for Evaluating Characteristics>

Next, the structure of the transistor of one embodiment of the presentinvention is described, and evaluation results of electricalcharacteristics of the transistor which was fabricated are shown.

Structure Example 1 of Transistor

FIG. 17A is a top view of a transistor 100A. FIG. 17B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.17A. FIG. 17C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 17A. For clarity, FIG. 17A does not illustrate somecomponents such as an insulating film 110. As in FIG. 17A, somecomponents are not illustrated in some cases in top views of transistorsdescribed below. Furthermore, the direction of the dashed-dotted lineX1-X2 may be referred to as a channel length (L) direction, and thedirection of dashed-dotted line Y1-Y2 may be referred to as a channelwidth (W) direction.

The transistor 100A illustrated in FIGS. 17A to 17C includes aconductive film 106 over a substrate 102; an insulating film 104 overthe conductive film 106; an oxide semiconductor film 108 over theinsulating film 104; the insulating film 110 over the oxidesemiconductor film 108; a conductive film 112 over the insulating film110; and an insulating film 116 over the insulating film 104, the oxidesemiconductor film 108, and the conductive film 112. Note that the oxidesemiconductor film 108 includes a channel region 108 i overlapping withthe conductive film 112, a source region 108 s in contact with theinsulating film 116, and a drain region 108 d in contact with theinsulating film 116.

Furthermore, the insulating film 116 contains nitrogen or hydrogen. Theinsulating film 116 is in contact with the source region 108 s and thedrain region 108 d, so that nitrogen or hydrogen that is contained inthe insulating film 116 is added to the source region 108 s and thedrain region 108 d. The source region 108 s and the drain region 108 deach have a high carrier density when nitrogen or hydrogen is addedthereto.

The transistor 100A may further include an insulating film 118 over theinsulating film 116, a conductive film 120 a electrically connected tothe source region 108 s through an opening portion 141 a provided in theinsulating films 116 and 118, and a conductive film 120 b electricallyconnected to the drain region 108 d through an opening portion 141 bprovided in the insulating films 116 and 118. In addition, an insulatingfilm 122 may be provided over the insulating film 118, the conductivefilm 120 a, and the conductive film 120 b. Although the structure wherethe insulating film 122 is provided is shown in FIGS. 17B and 17C, oneembodiment of the present invention is not limited thereto, and theinsulating film 122 is not necessarily provided.

In this specification and the like, the insulating film 104 may bereferred to as a first insulating film, the insulating film 110 may bereferred to as a second insulating film, the insulating film 116 may bereferred to as a third insulating film, the insulating film 118 may bereferred to as a fourth insulating film, and the insulating film 122 maybe referred to as a fifth insulating film. The insulating film 104functions as a first gate insulating film, and the insulating film 110functions as a second gate insulating film. The insulating films 116 and118 function as a protective insulating film and the insulating film 122functions as a planarization insulating film.

The insulating film 110 includes an excess oxygen region. Since theinsulating film 110 includes the excess oxygen region, excess oxygen canbe supplied to the channel region 108 i included in the oxidesemiconductor film 108. As a result, oxygen vacancies that might beformed in the channel region 108 i can be filled with excess oxygen,which can provide a highly reliable semiconductor device.

To supply excess oxygen to the oxide semiconductor film 108, excessoxygen may be supplied to the insulating film 104 that is formed underthe oxide semiconductor film 108. In that case, excess oxygen containedin the insulating film 104 might also be supplied to the source region108 s and the drain region 108 d included in the oxide semiconductorfilm 108. When excess oxygen is supplied to the source region 108 s andthe drain region 108 d, the resistance of the source region 108 s andthe drain region 108 d might be increased.

In contrast, in the structure in which the insulating film 110 formedover the oxide semiconductor film 108 contains excess oxygen, excessoxygen can be selectively supplied only to the channel region 108 i.Alternatively, the carrier density of the source and drain regions 108 sand 108 d can be selectively increased after excess oxygen is suppliedto the channel region 108 i and the source and drain regions 108 s and108 d, in which case an increase in the resistance of the source anddrain regions 108 s and 108 d can be prevented.

Furthermore, each of the source region 108 s and the drain region 108 dincluded in the oxide semiconductor film 108 preferably contains anelement that forms an oxygen vacancy or an element that is bonded to anoxygen vacancy. Typical examples of the element that forms an oxygenvacancy or the element that is bonded to an oxygen vacancy includehydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur,chlorine, titanium, and a rare gas. Typical examples of the rare gaselement are helium, neon, argon, krypton, and xenon. The element thatforms an oxygen vacancy is diffused from the insulating film 116 to thesource region 108 s and the drain region 108 d in the case where theinsulating film 116 contains one or more such elements. In addition oralternatively, the element that forms an oxygen vacancy is added to thesource region 108 s and the drain region 108 d by impurity additiontreatment.

An impurity element added to the oxide semiconductor film cuts a bondbetween a metal element and oxygen in the oxide semiconductor film, sothat an oxygen vacancy is formed. Alternatively, when the impurityelement is added to the oxide semiconductor film, oxygen bonded to ametal element in the oxide semiconductor film is bonded to the impurityelement, and the oxygen is released from the metal element, whereby anoxygen vacancy is formed. As a result, the oxide semiconductor film hasa higher carrier density and thus the conductivity thereof becomeshigher.

The conductive film 106 functions as a first gate electrode and theconductive film 112 functions as a second gate electrode. The conductivefilm 120 a functions as a source electrode and the conductive film 120 bfunctions as a drain electrode.

As illustrated in FIG. 17C, an opening portion 143 is provided in theinsulating films 104 and 110. The conductive film 106 is electricallyconnected to the conductive film 112 through the opening portion 143.Thus, the same potential is applied to the conductive film 106 and theconductive film 112. Note that different potentials may be applied tothe conductive film 106 and the conductive film 112 without providingthe opening portion 143. Alternatively, the conductive film 106 may beused as a light-shielding film without providing the opening portion143. When the conductive film 106 is formed using a light-shieldingmaterial, for example, light irradiating the channel region 108 i fromthe bottom can be reduced.

As illustrated in FIGS. 17B and 17C, the oxide semiconductor film 108faces the conductive film 106 functioning as a first gate electrode andthe conductive film 112 functioning as a second gate electrode and ispositioned between the two conductive films functioning as the gateelectrodes.

Furthermore, the length of the conductive film 112 in the channel widthdirection is larger than the length of the oxide semiconductor film 108in the channel width direction. In the channel width direction, thewhole oxide semiconductor film 108 is covered with the conductive film112 with the insulating film 110 placed therebetween. Since theconductive film 112 is connected to the conductive film 106 through theopening portion 143 provided in the insulating films 104 and 110, a sidesurface of the oxide semiconductor film 108 in the channel widthdirection faces the conductive film 112 with the insulating film 110placed therebetween.

In other words, in the channel width direction of the transistor 100A,the conductive films 106 and 112 are connected to each other through theopening portion 143 provided in the insulating films 104 and 110, andthe conductive films 106 and 112 surround the oxide semiconductor film108 with the insulating films 104 and 110 placed therebetween.

Such a structure enables the oxide semiconductor film 108 included inthe transistor 100A to be electrically surrounded by electric fields ofthe conductive film 106 functioning as a first gate electrode and theconductive film 112 functioning as a second gate electrode. A devicestructure of a transistor, like that of the transistor 100A, in whichelectric fields of the first gate electrode and the second gateelectrode electrically surround the oxide semiconductor film 108 inwhich a channel region is formed can be referred to as a surroundedchannel (s-channel) structure. Note that the transistor 100A can also becalled a dual-gate transistor owing to the number of its gateelectrodes.

Since the transistor 100A has the S-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 106 or the conductive film112; thus, the current drive capability of the transistor 100A can beimproved and high on-state current characteristics can be obtained.Since the on-state current can be increased, it is possible to reducethe size of the transistor 100A. Furthermore, since the transistor 100Ahas a structure in which the oxide semiconductor film 108 is surroundedby the conductive film 106 and the conductive film 112, the mechanicalstrength of the transistor 100A can be increased.

Note that in the channel width direction of the transistor 100A, anopening portion which is different from the opening portion 143 may beformed on the side of the oxide semiconductor film 108 on which theopening portion 143 is not formed.

Note that the transistor 100A can also be called a top-gate self-aligned(TGSA) FET, owing to the position of the conductive film 112 withrespect to the oxide semiconductor film 108 and a formation method ofthe conductive film 112. Note that the semiconductor device of oneembodiment of the present invention is not limited thereto and may be abottom-gate top-contact (BGTC) FET.

[Formation of Transistor]

Next, transistors which correspond to the above-described transistor100A were formed and the electrical characteristics of the transistorswere evaluated. In this embodiment, Samples S1A to S1C described belowwere fabricated. Samples S1A to S1C are each a sample in which atransistor with a channel length L of 2 μm and a channel width W of 3 μmis formed. Sample S1A includes the composite oxide semiconductor shownin FIGS. 4A and 4B as the oxide semiconductor film 108, Sample S1Bincludes the composite oxide semiconductor shown in FIGS. 5A and 5B asthe oxide semiconductor film 108, and Sample S1C includes the compositeoxide semiconductor shown in FIGS. 6A and 6B as the oxide semiconductorfilm 108.

[Method for Forming Samples S1A to S1C]

First, a 10-nm-thick titanium film and a 100-nm-thick copper film wereformed over a glass substrate with a sputtering apparatus. Next, theconductive film was processed by a photolithography method.

A stack including four insulating films was formed over the substrateand the conductive film. The insulating films were formed in successionin a vacuum with a plasma-enhanced chemical deposition (PECVD)apparatus. As the insulating films, a 50-nm-thick silicon nitride film,a 300-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film,and a 50-nm-thick silicon oxynitride film were used and stacked in thisorder.

Next, an oxide semiconductor film was formed over the insulating filmand was processed into an island shape, whereby a semiconductor layerwas formed. A 40-nm-thick oxide semiconductor film was formed as theoxide semiconductor film 108. Note that the oxide semiconductor film isthe above-described composite oxide semiconductor or C/IGZO.

The oxide semiconductor film of Sample S1A was formed under thefollowing conditions: the substrate temperature was room temperature(25° C.); an argon gas with a flow rate of 180 sccm and an oxygen gaswith a flow rate of 20 sccm were introduced into a chamber of thesputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of2.5 kw was applied to a metal oxide target containing indium, gallium,and zinc (In:Ga:Zn=4:2:4.1 [atomic ratio]). The oxygen flow ratepercentage in deposition of the oxide semiconductor film of Sample S1Awas 10%.

Next, an insulating film was formed over the insulating film and theoxide semiconductor layer. As the insulating film, a 150-nm-thicksilicon oxynitride film was formed with a PECVD apparatus.

Next, heat treatment was performed. The heat treatment was performed at350° C. in a mixed gas atmosphere of nitrogen and oxygen for one hour.

An opening portion was formed in a desired region of the insulatingfilm. The opening portion was formed by a dry etching method.

Then, a 100-nm-thick oxide semiconductor film was formed over theinsulating film and in the opening portion and the oxide semiconductorfilm was processed into an island shape, whereby a conductive film wasformed. In addition, the insulating film in contact with the bottomsurface of the conductive film was processed in succession after theformation of the conductive film, whereby the insulating film wasformed.

As the conductive film, a 10-nm-thick oxide semiconductor film, a50-nm-thick titanium nitride film, and a 100-nm-thick copper film wereformed in this order. The oxide semiconductor film was formed under thefollowing conditions: the substrate temperature was 170° C.; an oxygengas with a flow rate of 200 sccm was introduced into a chamber of thesputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of2.5 kw was applied to a metal oxide target containing indium, gallium,and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1). The titaniumnitride film and the copper film were formed with a sputteringapparatus.

Then, plasma treatment was performed from above the oxide semiconductorfilm, the insulating film, and the conductive film. The plasma treatmentwas performed with a PECVD apparatus at a substrate temperature of 220°C. in a mixed gas atmosphere containing an argon gas and a nitrogen gas.

Then, an insulating film was formed over the oxide semiconductor film,the insulating film, and the conductive film. The insulating film wasformed by stacking a 100-nm-thick silicon nitride film and a300-nm-thick silicon oxynitride film with a PECVD apparatus.

Then, a mask was formed over the formed insulating film and an openingportion was formed in the insulating film with use of the mask.

A conductive film was formed to fill the opening portion and wasprocessed into an island shape, whereby the conductive films serving asa source electrode and a drain electrode was formed. For the conductivefilms, a 10-nm-thick titanium film and a 100-nm-thick copper film wereformed with a sputtering apparatus.

After that, an insulating film was formed over the insulating film andthe conductive films. A 1.5-μm-thick acrylic-based photosensitive resinwas used for the insulating film.

In such a manner, Sample S1A was formed.

Sample S1B was formed in a manner similar to the manner in which SampleS1A was formed, except the formation conditions of the oxidesemiconductor film. The oxide semiconductor film of Sample S1B wasformed under the following conditions: the substrate temperature wasroom temperature (25° C.); an argon gas with a flow rate of 100 sccm andan oxygen gas with a flow rate of 100 sccm were introduced into achamber of the sputtering apparatus; the pressure was set to 0.6 Pa; andan AC power of 2.5 kw was applied to a metal oxide target containingindium, gallium, and zinc (In:Ga:Zn=4:2:4.1 [atomic ratio]). The oxygenflow rate percentage in deposition of the oxide semiconductor film ofSample S1B was 50%.

Sample S1C was formed in a manner similar to the manner in which SampleS1A was formed, except the formation conditions of the oxidesemiconductor film. The oxide semiconductor film of Sample S1C wasformed under the following conditions: the substrate temperature wasroom temperature (25° C.); an oxygen gas with a flow rate of 200 sccmwas introduced into a chamber of the sputtering apparatus; the pressurewas set to 0.6 Pa; and an AC power of 2.5 kw was applied to a metaloxide target containing indium, gallium, and zinc (with an atomic ratioof In:Ga:Zn=4:2:4.1). The oxygen flow rate percentage in deposition ofthe oxide semiconductor film of Sample S1C was 100%.

[I_(d)−V_(g) Characteristics of Transistors]

Next, I_(d)−V_(g) characteristics of the transistors in Samples S1A toS1C were measured. As conditions for measuring the I_(d)−V_(g)characteristics of each transistor, a voltage applied to the conductivefilm serving as a first gate electrode (hereinafter the voltage is alsoreferred to as gate voltage (V_(g))) and a voltage applied to theconductive film serving as the second gate electrode (hereinafter thevoltage is also referred to as back gate voltage (V_(bg))) were changedfrom −10 V to +10 V in increments of 0.25 V. A voltage applied to theconductive film serving as a source electrode (the voltage is alsoreferred to as source voltage (Vs)) was 0 V (comm), and a voltageapplied to the conductive film serving as a drain electrode (the voltageis also referred to as drain voltage (V_(d))) was 0.1 V and 20 V.

FIG. 1 shows results of I_(d)−V_(g) characteristics of Sample S1A, FIG.2 shows results of I_(d)−V_(g) characteristics of Sample S1B, and FIG. 3shows results of I_(d)−V_(g) characteristics of Sample S1C. In FIG. 1 toFIG. 3, the first vertical axis represents I_(d) (A), the secondvertical axis represents field-effect mobility (μ_(FE)) (cm²/Vs), andthe horizontal axis represents V_(g) (V). Note that the field-effectmobility was measured when V_(d) was 20 V.

As shown in FIG. 1, the transistor that is a semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics. Table 2 shows the transistor characteristics shown inFIG. 1.

TABLE 2 μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/ [cm²V⁻¹s⁻¹] [V][V/decade] [A/cm²] [cm²V⁻¹s⁻¹] μFE(@Vg = 2 V) 65.9 0.95 0.09 <1 × 10⁻¹²44.2 1.49

As shown in Table 2, Sample S1A includes a first region where themaximum value of the field-effect mobility of the transistor at a gatevoltage of higher than 0 V and lower than or equal to 10 V is largerthan or equal to 40 cm²/Vs and smaller than 150 cm²/Vs, a second regionwhere the threshold voltage is higher than or equal to −1 V and lowerthan or equal to 1 V, a third region where the S value is smaller than0.3 V/decade, and a fourth region where the off-state current is lowerthan 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than orequal to 1 and smaller than 1.5 where μ_(FE)(max) represents the maximumvalue of the field-effect mobility of the transistor andμ_(FE)(V_(g)=2V) represents the value of the field-effect mobility ofthe transistor at a gate voltage of 2 V.

As shown in FIG. 2, the transistor that is a semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics. Table 3 shows the transistor characteristics shown inFIG. 2.

TABLE 3 μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/ [cm²V⁻¹s⁻¹] [V][V/decade] [A/cm²] [cm²V⁻¹s⁻¹] μFE(@Vg = 2 V) 70.5 −0.17 0.13 <1 × 10⁻¹²42.5 1.66

As shown in Table 3, Sample S1B includes a first region where themaximum value of the field-effect mobility of the transistor at a gatevoltage of higher than 0 V and lower than or equal to 10 V is largerthan or equal to 40 cm²/Vs and smaller than 150 cm²/Vs, a second regionwhere the threshold voltage is higher than or equal to −1 V and lowerthan or equal to 1 V, a third region where the S value is smaller than0.3 V/decade, and a fourth region where the off-state current is lowerthan 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than orequal to 1.5 and smaller than 3 where μ_(FE)(max) represents the maximumvalue of the field-effect mobility of the transistor andμ_(FE)(V_(g)=2V) represents the value of the field-effect mobility ofthe transistor at a gate voltage of 2 V.

As shown in FIG. 3, the transistor that is a semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics. Table 4 shows the transistor characteristics shown inFIG. 3.

TABLE 4 μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/ [cm²V⁻¹s⁻¹] [V][V/decade] [A/cm²] [cm²V⁻¹s⁻¹] μFE(@Vg = 2 V) 28.0 0.21 0.16 <1 × 10⁻¹²7.3 3.84

As shown in Table 4, Sample S1C includes a first region where themaximum value of the field-effect mobility of the transistor at a gatevoltage of higher than 0 V and lower than or equal to 10 V is largerthan or equal to 10 cm²/Vs and smaller than 100 cm²/Vs, a second regionwhere the threshold voltage is higher than or equal to −1 V and lowerthan or equal to 1 V, a third region where the S value is smaller than0.3 V/decade, and a fourth region where the off-state current is lowerthan 1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than orequal to 3 and smaller than 10 where μ_(FE)(max) represents the maximumvalue of the field-effect mobility of the transistor andμ_(FE)(V_(g)=2V) represents the value of the field-effect mobility ofthe transistor at a gate voltage of 2 V.

The transistor characteristics can be obtained by using theabove-described composite oxide semiconductor or C/IGZO. In thetransistor including the above-described composite oxide semiconductoror C/IGZO as the semiconductor layer, a function of achieving highcarrier mobility and a function of achieving excellent switchingcharacteristics can be obtained at the same time.

<1-12. Examination of Shape of Mobility Curve by Device Simulation>

Next, the shape of the mobility curve of the field-effect mobility ofthe transistor that is shown in FIG. 1 was examined by device simulationfrom various viewpoints.

In the device simulation, as factors which determine the shape of themobility curve, three factors were assumed: 1. Temperature dependence ofmobility; 2. Donor density distribution in channel region; and 3.Density of shallow defect states (also referred to as sDOS) in oxidesemiconductor film.

[1. Temperature Dependence of Mobility]

In a transistor including an oxide semiconductor film, the field-effectmobility is increased rapidly due to self-heating. The temperaturedependence of electron mobility (μ_(n)) of the oxide semiconductor filmis represented by the following formula (5).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack & \; \\{\mu_{n} = {\mu_{n\; 300}\left( \frac{T_{L}}{300} \right)}^{1.5}} & (5)\end{matrix}$

In the formula (5), μ_(n300) represents electron mobility of the oxidesemiconductor film at room temperature and TL represents latticetemperature. As shown in the formula (5), the field-effect mobility ofthe transistor including the oxide semiconductor film increases inproportion to approximately the temperature T to the power of 1.5.

[2. Donor Density Distribution in Channel Region]

Here, the effective channel lengths of the transistors of Samples S1A toSIC are described with reference to FIG. 18.

FIG. 18 is a schematic view illustrating a concept of an effectivechannel length of a transistor.

In FIG. 18, GE, GI, and OS refer to a gate electrode, a gate insulatingfilm, and an oxide semiconductor film, respectively. In the oxidesemiconductor film, an n-type region is formed. The effective channellength (L_(eff)) of the transistor is represented by the followingformula (6).[Formula 6]L _(eff) =L _(g)−2ΔL  (6)

In the formula (6), L_(g) represents a gate length and ΔL represents areduction width of a channel length.

The effective channel length of the transistor can be obtained bytransmission line model (TLM) analysis, for example.

In the following description, a model in which a donor density isgradually decreased from an n-type region to a channel region wasassumed on the basis of the above-described effective channel length.That is, donors are decreased toward the channel region in accordancewith the Gaussian distribution. FIGS. 19A to 19C show schematic viewsillustrating donor densities of channel regions.

FIGS. 19A, 19B, and 19C illustrate a donor density of Sample S1A, adonor density of Sample S1B, and a donor density of Sample S1C,respectively.

In FIGS. 19A to 19C, GE, GI, and OS refer to a gate electrode, a gateinsulating film, and an oxide semiconductor film, respectively. In theoxide semiconductor films illustrated in FIGS. 19A to 19C, a regionwhere a donor density is higher than or equal to 5×10¹⁸ cm³ is shown ingray and a region where a donor density is lower than or equal to 1×10¹⁶cm³ is shown in black.

From the results shown in FIGS. 19A, 19B, and 19C, the effective channellengths in the transistors shown in FIGS. 19A, 19B, and 19C wereestimated to be 2.0 μm, 1.2 μm, and 0.8 μm, respectively. In otherwords, ΔL of the transistor shown in FIG. 19A, ΔL of the transistorshown in FIG. 19B, and ΔL of the transistor shown in FIG. 19C wereestimated to be 0 μm, 0.4 μm, and 0.6 μm, respectively.

[3. Density of Shallow Defect States in Oxide Semiconductor Film]

Next, a density of shallow defect states (also referred to as sDOS) inan oxide semiconductor film is described. The sDOS of an oxidesemiconductor film can be estimated from electrical characteristics of atransistor including the oxide semiconductor film. In the descriptionbelow, the density of interface states of the transistor was measured.In addition, a method for estimating subthreshold leakage current inconsideration of the density of interface states and the number ofelectrons trapped by the interface states, N_(trap) is described.

The number of electrons trapped by the interface state, N_(trap), can bemeasured by comparing drain current-gate voltage (I_(d)−V_(g)) of thetransistor that was actually measured and drain current-gate voltage(I_(d)−V_(g)) characteristics that was calculated.

FIG. 20 illustrates ideal I_(d)−V_(g) characteristics obtained bycalculation and the actually measured I_(d)−V_(g) characteristics of thetransistor when a source voltage Vs is 0 V and a drain voltage V_(d) is0.1 V. Note that only values more than or equal to 1×10⁻¹³ A at whichdrain voltage I_(d) can be easily measured were plotted among themeasurement results of the transistor.

A change of the drain current I_(d) with respect to the gate voltageV_(g) is more gradual in the actually measured I_(d)−V_(g)characteristics than in the ideal I_(d)−V_(g) characteristics obtainedby calculation. This is probably because an electron is trapped by ashallow interface state positioned near energy at the bottom of theconduction band (represented as E_(C)). In this measurement, the densityof interface surface Nit can be estimated more accurately inconsideration of the number of electrons (per unit area and unit energy)trapped by a shallow interface state, N_(trap), with use of the Fermidistribution function.

First, a method for evaluating the number of electrons trapped by aninterface trap state, N_(trap), by using schematic I_(d)−V_(g)characteristics illustrated in FIG. 21 is described. The dashed lineindicates ideal I_(d)−V_(g) characteristics without trap state and isobtained by the calculation. On the dashed line, a change in gatevoltage V_(g) when the drain current changes from I_(d)1 to I_(d)2 isrepresented by ΔV_(id). The solid line indicates the actually measuredI_(d)−V_(g) characteristics. On the solid line, a change in gate voltageV_(g) when the drain current changes from I_(d)1 to I_(d)2 isrepresented by ΔV_(ex). The potential at the target interface when thedrain current is I_(d)1, the potential at the target interface when thedrain current is I_(d)2, and the amount of change are represented byϕ_(it1), ϕ_(it2), and Δϕ_(it), respectively.

The slope of the actually measured values is smaller than that of thecalculated values in FIG. 21, which indicates that ΔV_(ex) is alwayslarger than ΔV_(id). Here, a difference between ΔV_(ex) and ΔV_(id)corresponds to a potential difference that is needed for trapping of anelectron in a shallow interface state. Therefore, ΔQ_(trap) which theamount of change in charge due to trapped electrons can be expressed bythe following formula (7).[Formula 7]ΔQ _(trap) =−C _(tg)(ΔV _(ex) −ΔV _(id))  (7)

C_(tg) is combined capacitance of an insulator and a semiconductor perunit area. In addition, ΔQ_(trap) can be expressed by the formula (8) byusing the number of trapped electrons N_(trap) (per unit area and perunit energy). Note that q represents elementary charge.[Formula 8]ΔQ _(trap) =−qN _(trap)Δϕ_(it)  (8)

Simultaneously solving the formulae (7) and (8) gives the formula (9).[Formula 9]—C _(tg)(ΔV _(ex) −ΔV _(id))=−qN _(trap)Δϕ_(it)  (9)

Then, taking the limit zero of Δϕ_(it) in the formula (9) gives theformula (10).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack & \; \\{N_{trap} = {{\frac{C_{tg}}{q}{\lim\limits_{{\Delta\phi}_{it}\rightarrow 0}\left( {\frac{\Delta\; V_{ex}}{{\Delta\phi}_{it}} - \frac{\Delta\; V_{id}}{\Delta\;\phi_{it}}} \right)}} = {C_{tg}\left( {\frac{\partial V_{ex}}{\partial\phi_{it}} - \frac{\partial V_{id}}{\partial\phi_{it}}} \right)}}} & (10)\end{matrix}$

In other words, the number of electrons trapped by an interface surface,N_(trap), can be estimated by using the ideal I_(d)−V_(g)characteristics, the actually measured I_(d)−V_(g) characteristics, andthe formula (10). Note that the relationship between the drain currentand the potential at the interface surface can be obtained bycalculation with the device simulator described above.

The relationship between the number of electrons N_(trap) per unit areaand per unit energy and the density of interface surface Nit isexpressed by the formula (11).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack & \; \\{N_{trap} = {\frac{\partial}{\partial\phi_{it}}{\int_{- \infty}^{\infty}{{N_{it}(E)}{f(E)}{dE}}}}} & (11)\end{matrix}$

Here, f(E) is Fermi distribution function. The N_(trap) obtained fromthe formula (10) is fitted with the formula (11) to determine Nit. Theconduction characteristics including I_(d)<0.1 pA can be obtained by thedevice simulator to which the Nit is set.

The actually measured I_(d)−V_(g) characteristics in FIG. 22 is appliedto the formula (10) and the results of extracting N_(trap) are plottedas white circles in FIG. 20. The vertical axis in FIG. 22 representsFermi energy Ef at the bottom of the conduction band E_(C) of asemiconductor. The maximum value is positioned on the dashed line justunder E_(C). When tail distribution of the formula (12) is assumed asNit of the formula (11), N_(trap) can be fitted well like the dashedline in FIG. 22. As a result, the peak value N_(ta)=1.67×10¹³ cm⁻² eV⁻¹and the characteristic width W_(ta)=0.105 eV are obtained as the fittingparameters.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack & \; \\{{N_{it}(E)} = {N_{ta}\mspace{11mu}{\exp\;\left\lbrack \frac{E - E_{c}}{W_{ta}} \right\rbrack}}} & (12)\end{matrix}$

FIGS. 23A and 23B show the inverse calculation results of I_(d)−V_(g)characteristics by feeding back the obtained fitting curve of interfacestate to the calculation using the device simulator. FIG. 23A shows thecalculated I_(d)−V_(g) characteristics when the drain voltage V_(d) is0.1 V and 1.8V and the actually measured I_(d)−V_(g) characteristicswhen the drain voltage V_(d) is 0.1 V and 1.8V. FIG. 23B is a graph inwhich the drain current I_(d) is a logarithm in FIG. 23A.

The curve obtained by the calculation substantially matches with theplot of the actually measured values, which suggests that the calculatedvalues and the measured values are highly reproducible. Thus, the abovemethod is quite appropriate as a method for calculating the density ofshallow defect states.

[4. Calculation Results of Mobility Curve]

The sDOS in the oxide semiconductor film influences the mobility curveof the field-effect mobility. In particular, in the vicinity of thethreshold voltage, the shape of the mobility curve is changed becauseelectrons are trapped by the sDOS. The sDOS in the oxide semiconductorfilm is represented by the product of N_(ta) and W_(ta) in the formula(12) and the thickness of t_(OS). Then, the mobility curve wascalculated on the basis of the formula (12). Table 5 shows parametersused for the calculation.

TABLE 5 Structure L length 2 μm W width 3 μm S/D electrode Material Cu —Work function 4.4 eV Thickness 110 μm Directly under n+ 5.00E+18 cm⁻³S/D and entire Loff region Passivation film Material SiON — Relative 3.9— permittivity ε Thickness 400 nm Material SiN — Relative 7.5 —permittivity ε Thickness 50 nm Upper Material W — gate electrode Workfunction 5.0 — Thickness 110 nm Upper GI Material SiON — Relative 3.9 —permittivity ε Thickness 150 nm OS Composition IGZO(4:2:4.1) — formulaElectron affmity 4.4 eV Eg 3.0 eV Relative 15 — permittivity ε Donordensity 6.60E−09 cm⁻³ Electron mobility 15 cm²/Vs Hole mobility 0.01cm²/Vs Nc 5.00E+18 cm⁻³ Nv 5.00E+18 cm⁻³ Thermal 0.014 W/cmKconductivity Thickness 40 nm Lower GI Material SiON — Relative 3.9 —permittivity ε Thickness 50 nm Material SiN — Relative 7.5 —permittivity ε Thickness 400 — Lower Material W — gate electrode Workfunction 5.0 — Thickness 110 nm

In this embodiment, the mobility curve in the case where the value ofW_(ta) was varied was calculated. FIG. 24 shows the shapes of themobility curves with different values of W_(ta). In FIG. 24, N_(ta) is2.5×10¹⁹ cm³ eV¹ and ΔL is 0. In addition, there were seven conditionswhere W_(ta) was 0.015 eV, 0.02 eV, 0.025 eV, 0.03 eV, 0.035 eV, 0.04eV, and 0.045 eV.

As shown in FIG. 24, as the value of W_(ta) is smaller, that is, as theenergy width of the sDOS is narrower, the mobility curve rises moresteeply. In addition, it is found that as the energy width of the sDOSis narrower, the peak value of the mobility curve is shifted from thehigh V_(g) side to the low V_(g) side and reduced.

[5. Influence of Diffusion Current on Mobility Curve]

Next, the influence of diffusion current on a mobility curve isdescribed. The drain current of a FET is represented by the followingformula (13).[Formula 13]j _(n) =qnμ _(n) E+qD _(n) V _(n)  (13)

As shown in the formula (13), the drain current is the sum of thecomponent of drift current and the component of diffusion current. Notethat in the formula (13), the first term and the second term representdrift current and diffusion current, respectively. FIG. 25 is aschematic view illustrating the contribution of the components ofdiffusion current and drift current in the I_(d)−V_(g) characteristics.

The influence of the component of diffusion current on a mobility curve(saturation) was estimated by device simulation. As a FET, a dual-gateFET in which an active layer was an oxide semiconductor, source anddrain regions were n⁺ regions, and a channel region was intrinsic wasassumed. Table 6 shows the calculation conditions. Note that trap states(e.g., sDOS) at the interface between a GI and the active layer and inthe active layer are not assumed.

TABLE 6 Channel length  6 μm Channel width  50 μm Thickness of OS film 35 nm Thickness of GI film 256 nm Thickness of passivation film 480 nmMobility of OS 10 (cm²/Vsec) Donor density 6.6E−9 (1/cm³) in channelportion Donor density 1.0E+19 (1/cm³) directly under S/D Drain voltage0.1 V, 10 V

FIG. 26 shows I_(d)−V_(g) characteristics and a saturation-mobilitycurve obtained by the simulation. Note that in FIG. 26, the I_(d)−V_(g)characteristics and the saturation-mobility curve overlap with eachother. As shown in FIG. 26, the mobility curve has a peak near V_(th) ofthe I_(d)−V_(g) characteristics.

FIG. 27 is a schematic band diagram of the dual-gate FET in thethickness direction.

As shown in FIG. 27, the band is comparatively flat in the thicknessdirection of the semiconductor owing to a gate electric field andcurrent thus flows in the whole film of the semiconductor. In adual-gate FET having such a band, the drain current in a subthresholdregion is represented by the following formula (14).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack & \; \\{I_{d} = {\frac{W}{L}\mu\;{kTn}_{i}t_{S}\mspace{11mu}\exp\mspace{11mu}{\left( \frac{{- q}\;\Delta\;\phi}{kT} \right)\left\lbrack {{\exp\mspace{11mu}\left( \frac{{qV}_{g}}{kT} \right)} - {\exp\mspace{11mu}\left( \frac{q\left( V_{th} \right)}{kT} \right)}} \right\rbrack}}} & (14)\end{matrix}$

In the formula (14), k represents the Boltzmann constant, T representsthe temperature, n_(i) represents the intrinsic carrier density, t_(S)represents the thickness of the active layer, and Δϕ represents thedifference between the intrinsic Fermi level and the work function ofthe gate. The formula (14) implies that the diffusion current isproportional to the difference between the diffusion current density atthe end portion of the source region and the diffusion current densityat the end portion of the drain region.

By substituting the formula (14) into the formula defining thesaturation mobility, a formula (15) can be obtained.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 15} \right\rbrack} & \; \\{{\mu_{FE}^{sat} \equiv {\left( \frac{d\sqrt{I_{d}}}{{dV}_{g}} \right)^{2}\frac{2L}{C_{OX}W}}} = {\mu\frac{q^{2}n_{i}t_{S}\mspace{11mu}\exp\mspace{11mu}\left( {{- q}\;\Delta\;{\phi/{kT}}} \right)}{C_{OX}{kT}}\frac{\exp\mspace{11mu}\left( {{qV}_{g}/{kT}} \right)}{1 - {\exp\mspace{11mu}\left( {{q\left( {V_{th} - V_{g}} \right)}/{kT}} \right)}}}} & (15)\end{matrix}$

According to the formula (15), the saturation-mobility curve has a peaknear V_(th). That is, it is probable that the peak near V_(th) of thesaturation-mobility curve, which is observed in the ideal simulationwhere trap states such as sDOS are not assumed, is caused by thediffusion current flowing in the whole active layer.

However, in an actual mobility curve of an IGZO-FET, such a sharp peaknear V_(th) is not observed. When shallow electron trap states (i.e.,sDOS) are assumed to exist in the OS film or at the interface betweenthe OS film and the gate insulating film, the mobility curve can be madeclose to the shape of a measured mobility curve. FIG. 28 shows thedevice simulation results where sDOS is assumed in the OS film.

FIG. 28 suggests that the mobility curve is influenced by sDOS in the OSfilm. The amount of sDOS in the OS film increases with an increase in OSfilm thickness. Therefore, the field-effect mobility decreases with anincrease in OS film thickness. FIG. 29 shows the relation between thefield-effect mobility (maximum) and the OS film thickness of an FET.

In FIG. 29, the vertical axis represents the field-effect mobility(maximum) and the horizontal axis represents the OS film thickness.Furthermore, in FIG. 29, the results of transistors of four kindsdiffering in channel length L (L=2 μm, 3 μm, 6 μm, and 10 μm) overlapwith each other. As shown in FIG. 29, the field-effect mobilitydecreases with an increase in OS film thickness.

FIGS. 30A and 30B show saturation-mobility curves calculated under theconditions of different distribution of sDOS. FIG. 30A shows thedistribution of sDOS in OS films and FIG. 30B shows the shapes ofmobility curves. As shown by the arrows in FIGS. 30A and 30B, the shapeof saturation-mobility curve changes with the amount of the energylevels of sDOS or the distribution of sDOS.

[6. Influence of Parasitic Resistances of Source Region and DrainRegion]

Next, the influence of parasitic resistances of a source region and adrain region (also referred to as SD regions) of a TGSA OS-FET isdescribed. A TGSA OS-FET includes, on both sides of a channel region, asource region and a drain region formed by reducing the resistance of anactive layer. There is a case where the source region and the drainregion behave as parasitic resistances in FET characteristics. The TGSAOS-FET in this case is represented by a circuit diagram as in FIG. 31A.

In the case of the circuit diagram shown in FIG. 31A, V_(d) is dividedinto voltages applied to two parasitic resistances and a voltage appliedto the FET as shown in a formula (16).[Formula 16]V _(d) =RI _(d) +V _(FET) +RI _(d)  (16)

In the formula (16), R represents parasitic resistance and V_(FET)represents the potential difference applied to both ends of the channelof the FET. In a GCA formula, a region higher than or equal to V_(g)that satisfies V_(FET)=V_(g)−V_(th) is considered as a linear region.Therefore, in the case where a parasitic resistance exists as shown inFIG. 31A, V_(g) at which the linear region is formed, i.e., V_(g) thatsatisfies V_(FET)=V_(g)−V_(th), is lower than that in the case where noparasitic resistance exists.

As shown in FIG. 15, the saturation mobility decreases after enteringthe linear region. In light of this, it is assumed that V_(g) at thelower limit of the linear region is decreased with an increase in theresistance value of a parasitic resistance. Hence, the relation betweenthe field-effect mobility and V_(g) was obtained by device simulation inwhich the sizes of the source region and the drain region were changed.FIG. 31B shows the device simulation results. As shown in FIG. 31B, byincreasing the sizes of the source region and the drain region formed byreducing the resistance of the active layer to increase parasiticresistances, V_(g) at the lower limit of the linear region, i.e., V_(g)at which the saturation-mobility curve starts decreasing, is lowered.

[7. Influence of Self-Generated Heat]

Next, the influence of self-generated heat of a FET is described. A FETgenerates heat when current flows in the FET. The amount of generatedheat increases with an increase in the amount of current, which is clearfrom Joule's law. Furthermore, the amount of current in a FET increaseswith an increase in V_(g), and self-generated heat of a FET increaseswith an increase in V_(g) according to Joule's law.

According to the above-described GCA, drain current in a saturationregion is proportional to (V_(g)−V_(th))² as shown in Formula (2).Therefore, when the square roots of drain current are plotted, a linearshape is obtained in the saturation region. The slope of the linearportion is calculated by differentiation, and the calculation resultsare normalized by a channel length, a channel width, and a gatecapacitance to convert them into mobility; thus, the saturation mobilityis obtained.

An obvious variable of temperature is not included in the GCA formula,so it appears that the saturation mobility does not depend ontemperature. However, in an OS-FET, some of parameters included in theGCA have temperature dependence. Therefore, the shape of thesaturation-mobility curve is changed by the influence of self-generatedheat of the FET.

The first of the parameters that change depending on temperature is theelectron mobility of an IGZO. The electron mobility of an IGZO isincreased with a temperature increase. That is, the electron mobility ofan IGZO is increased if the amount of current flowing in a FET isincreased and the temperature of the FET is increased. Therefore, underthe conditions where V_(g) or V_(d) is high, the temperature of a FET ishigh due to self-generated heat, so that the electron mobility isincreased and the drain current is increased. Consequently, the slope ofthe √I_(d)−V_(g) characteristics in the saturation region is increased,and the saturation mobility is increased. The influence of theself-generated heat of a FET is examined by device simulation. FIG. 32shows the device simulation results. In FIG. 32, the calculation resultsof the I_(d)−V_(g) characteristics and the saturation-mobility curve ofa TGSA CAAC-OS-FET are shown.

The influence of the self-generated heat of a FET is noticeableparticularly in a TGSA FET. This is because a TGSA FET is less likely torelease heat than a BGTC FET.

A BGTC FET has an excellent heat dissipation property because a sourceelectrode and a drain electrode are positioned near a channel regionwhere heat is generated in a BGTC FET. In contrast, a TGSA FET has a lowheat dissipation property because an electrode serving as a heatdissipation path is apart from a channel region in a TGSA FET. Thus, thetemperature of a TGSA FET is easily increased by self-generated heat,and the mobility curve is easily influenced by the self-generated heat.

The second of the parameters that change depending on temperature is thenumber of carrier electrons. The number of electrons per unit area thatare accumulated in a gate capacitance is represented byC_(OX)(V_(g)−V_(th)). In the case where electron traps exist, some ofthe accumulated electrons are trapped by the electron traps, and carrierelectrons are decreased accordingly. As described above, an OS includeselectron traps, sDOS; thus, some of electrons accumulated in the gatecapacitance do not serve as carriers.

The sDOS level is lower than the lower end of the conduction band;therefore, the proportion of carrier electrons to trapped electrons isincreased with a temperature increase when Boltzmann distribution istaken into consideration. Since the temperature of a FET is increasedwith a V_(g) increase as described above, the proportion of carrierelectrons is increased with a V_(g) increase. Accordingly, thesaturation mobility is also increased with a V_(g) increase.

The temperature dependence of the saturation mobility of a FET wascalculated by device simulation on the assumption that sDOS is takeninto consideration and the electron mobility of a CAAC-OS does notdepend on temperature. The calculation results are shown in FIG. 33. Asshown in FIG. 33, the mobility increases with a V_(g) increase moreclearly in the case where the electron mobility is dependent ontemperature than in the case where the electron mobility is independentof temperature.

[8. Influence of Reduction in Effective Channel Length]

The channel length of a CAAC-OS FET corresponds to the distance betweena source electrode and a drain electrode in a BGTC structure and thelength of a gate electrode in a TGSA structure. In actual FETcharacteristics, however, an effective channel length is the distancebetween n⁺ regions of a source region and a drain region. In accordancewith process conditions, the boundary between an n⁺ region and a channelregion is not aligned with a gate electrode end; in some cases, the n⁺region extends beyond the gate electrode end in a channel direction. Inthis case, the field-effect mobility is increased apparently. FIG. 34shows the relation between field-effect mobility and V_(g). In FIG. 34,the vertical axis represents field-effect mobility and the horizontalaxis represents V_(g).

In the case where the description so far is assumed, three kinds ofshapes of saturation-mobility curves of TGSA OS-FETs are obtained. FIGS.35A to 35C show calculation results of the saturation mobility of theTGSA OS-FETs. FIG. 35A corresponds to Sample S1A, FIG. 35B correspondsto Sample S1B, and FIG. 35C corresponds to Sample SIC.

As shown in FIGS. 35A to 35C, the shape of the saturation-mobility curveis changed variously, in particular, by setting a parameter of sDOS to aproper value. In the case of a semiconductor device corresponding toSample S1A, the saturation-mobility curve has a shape like that shown inFIG. 35A, which suggests that a sDOS value is small. Similarly, in thecase of a semiconductor device corresponding to Sample S1B, thesaturation-mobility curve has a shape like that shown in FIG. 35B, whichsuggests that a sDOS value is small. In the case of a semiconductordevice corresponding to Sample SIC, the saturation-mobility curve has ashape like that shown in FIG. 35C.

An sDOS value of the oxide semiconductor film of Sample S1A was measuredusing the I_(d)−V_(g) characteristics of the transistor that are shownin FIG. 1. In the measurement results, sDOS of the oxide semiconductorfilm of Sample S1A was 6.4×10⁻¹² cm². Thus, the oxide semiconductor filmof one embodiment of the present invention includes a region with asmall sDOS value, i.e., a region where density of shallow defect statesis lower than 1.0×10⁻¹² cm².

An sDOS value of the oxide semiconductor film of Sample S1B was measuredusing the I_(d)−V_(g) characteristics of the transistor that are shownin FIG. 2. In the measurement results, sDOS of the oxide semiconductorfilm of Sample S1B was 1.7×10⁻¹² cm². Thus, the oxide semiconductor filmof one embodiment of the present invention includes a region with asmall sDOS value, i.e., a region where density of shallow defect statesis higher than or equal to 1.0×10⁻¹² cm² and lower than 2.0×10⁻¹² cm².

An sDOS value of the oxide semiconductor film of Sample SIC was measuredusing the I_(d)−V_(g) characteristics of the transistor that are shownin FIG. 3. In the measurement results, sDOS of the oxide semiconductorfilm of Sample SIC was 2.4×10⁻¹² cm². Thus, the oxide semiconductor filmof one embodiment of the present invention includes a region with asmall sDOS value, i.e., a region where density of shallow defect statesis higher than or equal to 2.0×10⁻¹² cm² and lower than 3.0×10⁻¹² cm².

<1-13. Components of Transistor>

Next, details of the components of the transistor in FIGS. 17A to 17Cwill be described.

[Substrate]

A material having heat resistance high enough to withstand heattreatment in the manufacturing process can be used for the substrate102.

Specifically, non-alkali glass, soda-lime glass, potash glass, crystalglass, quartz, sapphire, or the like can be used. Alternatively, aninorganic insulating film may be used. Examples of the inorganicinsulating film include a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and an aluminum oxide film.

The non-alkali glass preferably has a thickness greater than or equal to0.2 mm and less than or equal to 0.7 mm, for example. The non-alkaliglass may be polished to obtain the above thickness.

Using the non-alkali glass, a large-sized glass substrate having any ofthe following sizes can be used: the 6th generation (1500 mm×1850 mm),the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, as the substrate 102, a single-crystal semiconductorsubstrate or a polycrystalline semiconductor substrate made of siliconor silicon carbide, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used.

Alternatively, an inorganic material such as metal may be used as thesubstrate 102. Examples of the inorganic material such as a metalinclude stainless steel or aluminum.

Alternatively, for the substrate 102, an organic material such as aresin, a resin film, or plastic may be used. Examples of the resin filminclude polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxyresin, polyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyether sulfone (PES), and a resin having a siloxane bond.

Alternatively, for the substrate 102, a composite material of acombination of an inorganic material and an organic material may beused. Examples of the composite material include a resin film to which ametal plate or a thin glass plate is bonded, a resin film into which afibrous or particulate metal or a fibrous or particulate glass isdispersed, and an inorganic material into which a fibrous or particulateresin is dispersed.

Note that the substrate 102 may be formed using one or more of aninsulating film, a semiconductor film, and a conductive film as long asit can at least support a film or a layer formed thereover andthereunder.

[First Insulating Film]

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. Forexample, the insulating film 104 can be formed to have a single-layerstructure or stacked-layer structure including an oxide insulating filmand/or a nitride insulating film. To improve the properties of theinterface with the oxide semiconductor film 108, at least a region ofthe insulating film 104 which is in contact with the oxide semiconductorfilm 108 is preferably formed using an oxide insulating film. When theinsulating film 104 is formed using an oxide insulating film from whichoxygen is released by heating, oxygen contained in the insulating film104 can be moved to the oxide semiconductor film 108 by heat treatment.

The thickness of the insulating film 104 can be greater than or equal to50 nm, greater than or equal to 100 nm and less than or equal to 3000nm, or greater than or equal to 200 nm and less than or equal to 1000nm. By increasing the thickness of the insulating film 104, the amountof oxygen released from the insulating film 104 can be increased, andinterface states at the interface between the insulating film 104 andthe oxide semiconductor film 108 and oxygen vacancies included in thechannel region 108 i of the oxide semiconductor film 108 can be reduced.

For example, the insulating film 104 can be formed to have asingle-layer structure or stacked-layer structure including siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,aluminum oxide, hafnium oxide, gallium oxide, a Ga—Zn oxide, or thelike. In this embodiment, the insulating film 104 has a stacked-layerstructure including a silicon nitride film and a silicon oxynitridefilm. With the insulating film 104 having such a stack-layer structureincluding a silicon nitride film as a lower layer and a siliconoxynitride film as an upper layer, oxygen can be efficiently introducedinto the oxide semiconductor film 108.

[Oxide Semiconductor Film]

As the oxide semiconductor film 108, the above-described composite oxidesemiconductor or C/IGZO is suitably used.

[Second Insulating Film]

The insulating film 110 has a function of supplying oxygen to the oxidesemiconductor film 108, particularly to the channel region 108 i. Theinsulating film 110 can be formed to have a single-layer structure or astacked-layer structure of an oxide insulating film or a nitrideinsulating film, for example. To improve the interface properties withthe oxide semiconductor film 108, a region which is in the insulatingfilm 110 and in contact with the oxide semiconductor film 108 ispreferably formed using at least an oxide insulating film. For example,silicon oxide, silicon oxynitride, silicon nitride oxide, or siliconnitride may be used for the insulating film 110.

The thickness of the insulating film 110 can be greater than or equal to5 nm and less than or equal to 400 nm, greater than or equal to 5 nm andless than or equal to 300 nm, or greater than or equal to 10 nm and lessthan or equal to 250 nm.

It is preferable that the insulating film 110 have few defects andtypically have as few signals observed by electron spin resonance (ESR)spectroscopy as possible. Examples of the signals include a signal dueto an E′ center observed at a g-factor of 2.001. Note that the E′ centeris due to the dangling bond of silicon. As the insulating film 110, asilicon oxide film or a silicon oxynitride film whose spin density of asignal due to the E′ center is lower than or equal to 3×10¹⁷ spins/cm³and preferably lower than or equal to 5×10¹⁶ spins/cm³ may be used.

In addition to the above-described signal, a signal due to nitrogendioxide (NO₂) might be observed in the insulating film 110. The signalis split into three signals according to nuclear spin of N; a firstsignal, a second signal, and a third signal. The first signal isobserved at a g-factor of greater than or equal to 2.037 and less thanor equal to 2.039. The second signal is observed at a g-factor ofgreater than or equal to 2.001 and less than or equal to 2.003. Thethird signal is observed at a g-factor of greater than or equal to 1.964and less than or equal to 1.966.

It is suitable to use an insulating film whose spin density due tonitrogen dioxide (NO₂) is higher than or equal to 1×10¹⁷ spins/cm³ andlower than 1×10¹⁸ spins/cm³ as the insulating film 110, for example.

Note that a nitrogen oxide (NO_(x)) such as a nitrogen dioxide (NO₂)forms a level in the insulating film 110. The level is positioned in theenergy gap of the oxide semiconductor film 108. Thus, when nitrogenoxide (NO_(x)) is diffused to the interface between the insulating film110 and the oxide semiconductor film 108, an electron might be trappedby the level on the insulating film 110 side. As a result, the trappedelectron remains in the vicinity of the interface between the insulatingfilm 110 and the oxide semiconductor film 108; thus, the thresholdvoltage of the transistor is shifted in the positive direction.Accordingly, the use of a film with a low nitrogen oxide content as theinsulating film 110 can reduce a shift of the threshold voltage of thetransistor.

As an insulating film that releases a small amount of nitrogen oxide(NO_(x)), for example, a silicon oxynitride film can be used. Thesilicon oxynitride film releases more ammonia than nitrogen oxide(NO_(x)) in thermal desorption spectroscopy (TDS); the typical releasedamount of ammonia is greater than or equal to 1×10¹⁸/cm³ and less thanor equal to 5×10¹⁹/cm³. Note that the released amount of ammonia is thetotal amount of ammonia released by heat treatment in a range from 50°C. to 650° C. or a range from 50° C. to 550° C. in TDS.

Since nitrogen oxide (NO_(x)) reacts with ammonia and oxygen in heattreatment, the use of an insulating film that releases a large amount ofammonia reduces nitrogen oxide (NO_(x)).

Note that in the case where the insulating film 110 is analyzed by SIMS,the nitrogen concentration in the film is preferably lower than or equalto 6×10²⁰ atoms/cm³.

The insulating film 110 may be formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), or hafnium oxide. The use of such a high-kmaterial enables a reduction in gate leakage current of a transistor.

[Third Insulating Film]

The insulating film 116 contains nitrogen or hydrogen. The insulatingfilm 116 may contain fluorine. The insulating film 116 is a nitrideinsulating film, for example. The nitride insulating film can be formedusing silicon nitride, silicon nitride oxide, silicon oxynitride,silicon nitride fluoride, silicon fluoronitride, or the like. Thehydrogen concentration in the insulating film 116 is preferably higherthan or equal to 1×10²² atoms/cm³. Furthermore, the insulating film 116is in contact with the source region 108 s and the drain region 108 d ofthe oxide semiconductor film 108. Thus, the concentration of an impurity(nitrogen or hydrogen) in the source region 108 s and the drain region108 d in contact with the insulating film 116 is increased, leading toan increase in the carrier density of the source region 108 s and thedrain region 108 d.

[Fourth Insulating Film]

As the insulating film 118, an oxide insulating film can be used.Alternatively, a stack including an oxide insulating film and a nitrideinsulating film can be used as the insulating film 118. The insulatingfilm 118 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide,gallium oxide, or Ga—Zn oxide.

Furthermore, the insulating film 118 preferably functions as a barrierfilm against hydrogen, water, and the like from the outside.

The thickness of the insulating film 118 can be greater than or equal to30 nm and less than or equal to 500 nm, or greater than or equal to 100nm and less than or equal to 400 nm.

[Fifth Insulating Film]

The insulating film 122 has an insulating property and is formed usingan inorganic material or an organic material. Examples of the inorganicmaterial include a silicon oxide film, a silicon oxynitride film, asilicon nitride oxide film, a silicon nitride film, an aluminum oxidefilm, and an aluminum nitride film. Examples of the organic materialinclude photosensitive resin materials such as an acrylic resin and apolyimide resin.

[Conductive Film]

The conductive films 106, 112, 120 a, and 120 b can be formed by asputtering method, a vacuum evaporation method, a pulsed laserdeposition (PLD) method, a thermal CVD method, or the like. Furthermore,as the conductive films 106, 112, 120 a, and 120 b, a conductive metalfilm, a conductive film that has a function of reflecting visible light,or a conductive film having a function of transmitting visible light maybe used.

A material containing a metal element selected from aluminum, gold,platinum, silver, copper, chromium, tantalum, titanium, molybdenum,tungsten, nickel, iron, cobalt, palladium, and manganese can be used forthe metal film having conductivity. Alternatively, an alloy containingany of the above metal elements may be used.

For the metal film having conductivity, specifically a two-layerstructure in which a copper film is stacked over a titanium film, atwo-layer structure in which a copper film is stacked over a titaniumnitride film, a two-layer structure in which a copper film is stackedover a tantalum nitride film, or a three-layer structure in which atitanium film, a copper film, and a titanium film are stacked in thisorder may be used. In particular, a conductive film containing a copperelement is preferably used because the resistance can be reduced. As anexample of the conductive film containing a copper element, an alloyfilm containing copper and manganese is given. The alloy film ispreferable because it can be processed by a wet etching method.

A tantalum nitride film is preferably used as each of the conductivefilms 106, 112, 120 a, and 120 b. Such a tantalum nitride film hasconductivity and a high barrier property against copper or hydrogen. Thetantalum nitride film can be used most preferably as a metal film incontact with the oxide semiconductor film 108 or a metal film in thevicinity of the oxide semiconductor film 108 because the amount ofhydrogen released from the tantalum nitride film is small.

As the conductive film having conductivity, a conductive macromoleculeor a conductive polymer may be used.

For the conductive film having a function of reflecting visible light, amaterial containing a metal element selected from gold, silver, copper,and palladium can be used. In particular, a conductive film containing asilver element is preferably used because reflectance of visible lightcan be improved.

For the conductive film having a function of transmitting visible light,a material containing an element selected from indium, tin, zinc,gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide,an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (alsoreferred to as ITSO), an In—Zn oxide, an In—Ga—Zn oxide, or the like canbe used.

As the conductive film having a function of transmitting visible light,a film containing graphene or graphite may be used. The film containinggraphene can be formed in the following manner: a film containinggraphene oxide is formed and is reduced. As a reducing method, a methodwith application of heat, a method using a reducing agent, or the likecan be employed.

The conductive films 112, 120 a, and 120 b can be formed by electrolessplating. As a material formed by the electroless plating, one or more ofCu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. Particularly, Cu or Agis preferable because the resistance of the conductive film can be low.

In the case where the conductive film is formed by electroless plating,a diffusion prevention film may be formed below the conductive film soas to prevent diffusion of constituent elements of the conductive filminto the outside. In addition, a seed layer that enables the conductivefilm to grow may be formed between the diffusion prevention film and theconductive film. The diffusion prevention film can be formed by, forexample, a sputtering method. As the diffusion prevention film, forexample, a tantalum nitride film or a titanium nitride film can be used.The seed layer can be formed by an electroless plating method.Alternatively, the seed layer can be formed using a material the same asa material of the conductive film that can be formed by an electrolessplating method.

Note that an oxide semiconductor typified by an In—Ga—Zn oxide may beused for the conductive film 112. The oxide semiconductor can have ahigh carrier density when nitrogen or hydrogen is supplied from theinsulating film 116. In other words, the oxide semiconductor functionsas an oxide conductor (OC). Accordingly, the oxide semiconductor can beused for a gate electrode.

The conductive film 112 can have, for example, a single-layer structureof an oxide conductor (OC), a single-layer structure of a metal film, ora stacked-layer structure of an oxide conductor (OC) and a metal film.

Note that it is suitable that the conductive film 112 has a single-layerstructure of a light-shielding metal film or a stacked-layer structureof an oxide conductor (OC) and a light-shielding metal film because thechannel region 108 i formed under the conductive film 112 can beshielded from light. In the case where the conductive film 112 has astacked-layer structure of an oxide semiconductor or an oxide conductor(OC) and a light-shielding metal film, formation of a metal film (e.g.,a titanium film or a tungsten film) over the oxide semiconductor or theoxide conductor (OC) produces any of the following effects: theresistance of the oxide semiconductor or the oxide conductor (OC) isreduced by the diffusion of the constituent element of the metal film tothe oxide semiconductor or oxide conductor (OC) side, the resistance isreduced by damage (e.g., sputtering damage) during the deposition of themetal film, and the resistance is reduced when oxygen vacancies areformed by the diffusion of oxygen in the oxide semiconductor or theoxide conductor (OC) to the metal film.

The thickness of the conductive films 106, 112, 120 a, and 120 b can begreater than or equal to 30 nm and less than or equal to 500 nm, orgreater than or equal to 100 nm and less than or equal to 400 nm.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 2

In this embodiment, a transistor in a mode different from that of thetransistor in Embodiment 1 is described with reference to FIG. 36A toFIG. 51C.

<2-1. Structure Example 2 of Transistor>

FIGS. 36A and 36B are cross-sectional views of a transistor 100B. FIGS.37A and 37B are cross-sectional views of a transistor 100C. FIGS. 38Aand 38B are cross-sectional views of a transistor 100D. The top views ofthe transistors 100B, 100C, and 100D are not illustrated because theyare similar to the top view of the transistor 100A in FIG. 17A.

The transistor 100B illustrated in FIGS. 36A and 36B is different fromthe transistor 100A in the layered structure of the conductive film 112,the shape of the conductive film 112, and the shape of the insulatingfilm 110.

The conductive film 112 in the transistor 100B includes a conductivefilm 112_1 over the insulating film 110 and the conductive film 112_2over the conductive film 112_1. For example, an oxide conductive film isused as the conductive film 112_1, so that excess oxygen can be added tothe insulating film 110. The oxide conductive film can be formed by asputtering method in an atmosphere containing an oxygen gas. As theoxide conductive film, an oxide film containing indium and tin, an oxidefilm containing tungsten and indium, an oxide film containing tungsten,indium, and zinc, an oxide film containing titanium and indium, an oxidefilm containing titanium, indium, and tin, an oxide film containingindium and zinc, an oxide film containing silicon, indium, and tin, oran oxide film containing indium, gallium, and zinc can be used, forexample.

As illustrated in FIG. 36B, the conductive film 112_2 is connected tothe conductive film 106 through the opening portion 143. By forming theopening portion 143 after a conductive film to be the conductive film112_1 is formed, the shape illustrated in FIG. 36B can be obtained. Inthe case where an oxide conductive film is used as the conductive film112_1, the structure in which the conductive film 112_2 is connected tothe conductive film 106 can decrease the contact resistance between theconductive film 112 and the conductive film 106.

The conductive film 112 and the insulating film 110 in the transistor100B have a tapered shape. More specifically, the lower end portion ofthe conductive film 112 is located outward from the upper end portion ofthe conductive film 112. The lower end portion of the insulating film110 is located outward from the upper end portion of the insulating film110. In addition, the lower end portion of the conductive film 112 isformed in substantially the same position as that of the upper endportion of the insulating film 110.

It is suitable that the conductive film 112 and the insulating film 110of the transistor 100B are formed to have tapered shapes because thecoverage with the insulating film 116 can be high as compared with thecase of the transistor 100A in which the conductive film 112 and theinsulating film 110 have rectangular shapes.

The other components of the transistor 100B are similar to those of thetransistor 100A described above and have similar effects.

The transistor 100C illustrated in FIGS. 37A and 37B is different fromthe transistor 100A in the layered structure of the conductive film 112,the shape of the conductive film 112, and the shape of the insulatingfilm 110.

The conductive film 112 in the transistor 100C includes the conductivefilm 112_1 over the insulating film 110 and the conductive film 112_2over the conductive film 112_1. A lower end portion of the conductivefilm 112_1 is located outward from an upper end portion of theconductive film 112_2. For example, the conductive film 112_1, theconductive film 112_2, and the insulating film 110 are processed withone mask, the conductive film 112_2 is processed by a wet etchingmethod, and the conductive film 112_1 and the insulating film 110 areprocessed by a dry etching method, whereby the above-described structurecan be obtained.

With the structure of the transistor 100C, regions 108 f are formed inthe oxide semiconductor film 108 in some cases. The regions 108 f areformed between the channel region 108 i and the source region 108 s andbetween the channel region 108 i and the drain region 108 d.

The regions 108 f function as high-resistance regions or low-resistanceregions. The high-resistance regions have the same level of resistanceas the channel region 108 i and do not overlap with the conductive film112 functioning as a gate electrode. In the case where the regions 108 fare high-resistance regions, the regions 108 f function as offsetregions. To suppress a decrease in the on-state current of thetransistor 100C, the regions 108 f functioning as offset regions mayeach have a length of 1 μm or less in the channel length (L) direction.

The low-resistance regions have a resistance that is lower than that ofthe channel region 108 i and higher than that of the source region 108 sand the drain region 108 d. In the case where the regions 108 f arelow-resistance regions, the regions 108 f function as lightly dopeddrain (LDD) regions. The regions 108 f functioning as LDD regions canrelieve an electric field in the drain region, thereby reducing a changein the threshold voltage of the transistor due to the electric field inthe drain region.

Note that in the case where the regions 108 f serve as LDD regions, forexample, the regions 108 f are formed by supplying one or more ofnitrogen, hydrogen, and fluorine from the insulating film 116 to theregions 108 f or by adding an impurity element from above the conductivefilm 112_1 using the insulating film 110 and the conductive film 112_1as a mask so that the impurity element is added to the oxidesemiconductor film 108 through the conductive film 112_1 and theinsulating film 110.

As illustrated in FIG. 37B, the conductive film 112_2 is connected tothe conductive film 106 through the opening portion 143.

The other components of the transistor 100C are similar to those of thetransistor 100A described above and have similar effects.

The transistor 100D illustrated in FIGS. 38A and 38B is different fromthe transistor 100A in the layered structure of the conductive film 112,the shape of the conductive film 112, and the shape of the insulatingfilm 110.

The conductive film 112 in the transistor 100D includes the conductivefilm 112_1 over the insulating film 110 and the conductive film 112_2over the conductive film 112_1. A lower end portion of the conductivefilm 112_1 is located outward from a lower end portion of the conductivefilm 112_2. Furthermore, a lower end portion of the insulating film 110is located outward from the lower end portion of the conductive film112_1. For example, the conductive film 112_1, the conductive film 1122,and the insulating film 110 are processed with one mask, the conductivefilm 112_2 and the conductive film 112_1 are processed by a wet etchingmethod, and the insulating film 110 is processed by a dry etchingmethod, whereby the above-described structure can be obtained.

As in the transistor 100C, the regions 108 f are formed in the oxidesemiconductor film 108 in the transistor 100D, in some cases. Theregions 108 f are formed between the channel region 108 i and the sourceregion 108 s and between the channel region 108 i and the drain region108 d.

As illustrated in FIG. 38B, the conductive film 112_2 is connected tothe conductive film 106 through the opening portion 143.

The other components of the transistor 100D are similar to those of thetransistor 100A described above and have similar effects.

<2-2. Structure Example 3 of Transistor>

FIGS. 39A and 39B are cross-sectional views of a transistor 100E. FIGS.40A and 40B are cross-sectional views of a transistor 100F. FIGS. 41Aand 41B are cross-sectional views of a transistor 100G. FIGS. 42A and42B are cross-sectional views of a transistor 100H. FIGS. 43A and 43Bare cross-sectional views of a transistor 100J. The top views of thetransistors 100E, 100F, 100G, 100H, and 100J are not illustrated becausethey are similar to the top view of the transistor 100A in FIG. 17A.

The transistors 100E, 100F, 100G, 100H, and 100J are different from theabove-described the transistor 100A in the structure of the oxidesemiconductor film 108. The other components are similar to those of thetransistor 100A and have similar effects.

The oxide semiconductor film 108 of the transistor 100E illustrated inFIGS. 39A and 39B includes an oxide semiconductor film 108_1 over theinsulating film 104, an oxide semiconductor film 108_2 over the oxidesemiconductor film 108_1, and an oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a three-layerstructure of the oxide semiconductor film 1081, the oxide semiconductorfilm 1082, and the oxide semiconductor film 108_3.

The oxide semiconductor film 108 of the transistor 100F illustrated inFIGS. 40A and 40B includes the oxide semiconductor film 108_2 over theinsulating film 104, and the oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a two-layer structureof the oxide semiconductor film 108_2 and the oxide semiconductor film108_3.

The oxide semiconductor film 108 of the transistor 100G illustrated inFIGS. 41A and 41B includes the oxide semiconductor film 108_1 over theinsulating film 104, and the oxide semiconductor film 108_2 over theoxide semiconductor film 108_1. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a two-layer structureof the oxide semiconductor film 108_1 and the oxide semiconductor film108_2.

The oxide semiconductor film 108 of the transistor 100H illustrated inFIGS. 42A and 42B includes the oxide semiconductor film 108_1 over theinsulating film 104, the oxide semiconductor film 108_2 over the oxidesemiconductor film 108_1, and the oxide semiconductor film 108_3 overthe oxide semiconductor film 108_2. The channel region 108 i has athree-layer structure of the oxide semiconductor film 108_1, the oxidesemiconductor film 108_2, and the oxide semiconductor film 108_3. Thesource region 108 s and the drain region 108 d each have a two-layerstructure of the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2. Note that in the cross section of thetransistor 100H in the channel width (W) direction, the oxidesemiconductor film 108_3 covers side surfaces of the oxide semiconductorfilm 108_1 and the oxide semiconductor film 108_2.

The oxide semiconductor film 108 of the transistor 100J illustrated inFIGS. 43A and 43B includes the oxide semiconductor film 108_2 over theinsulating film 104, and the oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i has a two-layerstructure of the oxide semiconductor film 108_2 and the oxidesemiconductor film 108_3. The source region 108 s and the drain region108 d each have a single-layer structure of the oxide semiconductor film108_2. Note that in the cross section of the transistor 100J in thechannel width (W) direction, the oxide semiconductor film 108_3 coversside surfaces of the oxide semiconductor film 108_2.

A side surface of the channel region 108 i in the channel width (W)direction or a region in the vicinity of the side surface is easilydamaged by processing, resulting in a defect (e.g., oxygen vacancy), oreasily contaminated by an impurity attached thereto. Therefore, evenwhen the channel region 108 i is substantially intrinsic, stress such asan electric field applied thereto activates the side surface of thechannel region 108 i in the channel width (W) direction or the region inthe vicinity of the side surface and turns it into a low-resistance(n-type) region easily. Moreover, if the side surface of the channelregion 108 i in the channel width (W) direction or the region in thevicinity of the side surface is an n-type region, a parasitic channelmay be formed because the n-type region serves as a carrier path.

Thus, in the transistor 100H and the transistor 100J, the channel region108 i has a stacked-layer structure and side surfaces of the channelregion 108 i in the channel width (W) direction are covered with onelayer of the stacked layers. With such a structure, defects on or in thevicinity of the side surfaces of the channel region 108 i can besuppressed or adhesion of an impurity to the side surfaces of thechannel region 108 i or to regions in the vicinity of the side surfacescan be reduced.

[Band Structure]

Here, a band structure of the insulating film 104, the oxidesemiconductor films 108_1, 1082, and 108_3, and the insulating film 110,a band structure of the insulating film 104, the oxide semiconductorfilms 108_2 and 108_3, and the insulating film 110, and a band structureof the insulating film 104, the oxide semiconductor films 108_1 and108_2, and the insulating film 110 will be described with reference toFIGS. 44A to 44C. Note that FIGS. 44A to 44C are each a band structureof the channel region 108 i.

FIG. 44A shows an example of a band structure in the thickness directionof a stack including the insulating film 104, the oxide semiconductorfilms 108_1, 1082, and 108_3, and the insulating film 110. FIG. 44Bshows an example of a band structure in the thickness direction of astack including the insulating film 104, the oxide semiconductor films108_2 and 108_3, and the insulating film 110. FIG. 44C shows an exampleof a band structure in the thickness direction of a stack including theinsulating film 104, the oxide semiconductor films 108_1 and 108_2, andthe insulating film 110. For easy understanding, the band structuresshow the conduction band minimum (E_(C)) of the insulating film 104, theoxide semiconductor films 108_1, 1082, and 108_3, and the insulatingfilm 110.

In the band structure of FIG. 44A, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:3:2 is used as the oxide semiconductor film 108_1, an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film108_2, and an oxide semiconductor film formed using a metal oxide targetwhose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxidesemiconductor film 108_3.

In the band structure of FIG. 44B, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is4:2:4.1 is used as the oxide semiconductor film 108_2, and an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film108_3.

In the band structure of FIG. 44C, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:3:2 is used as the oxide semiconductor film 108_1, and an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film108_2.

As illustrated in FIG. 44A, the conduction band minimum gradually variesbetween the oxide semiconductor films 1081, 108_2, and 108_3. Asillustrated in FIG. 44B, the conduction band minimum gradually variesbetween the oxide semiconductor films 108_2 and 108_3. As illustrated inFIG. 44C, the conduction band minimum gradually varies between the oxidesemiconductor films 108_1 and 108_2. In other words, the conduction bandminimum is continuously changed or continuously connected. To obtainsuch a band structure, there exists no impurity, which forms a defectstate such as a trap center or a recombination center, at the interfacebetween the oxide semiconductor films 108_1 and 108_2 or the interfacebetween the oxide semiconductor films 108_2 and 108_3.

To form a continuous junction between the oxide semiconductor films108_1, 108_2, and 108_3, it is necessary to form the films successivelywithout exposure to the air with a multi-chamber deposition apparatus(sputtering apparatus) provided with a load lock chamber.

With the band structure of FIG. 44A, FIG. 44B, or FIG. 44C, the oxidesemiconductor film 108_2 serves as a well, and a channel region isformed in the oxide semiconductor film 108_2 in the transistor with thestacked-layer structure.

By providing the oxide semiconductor films 108_1 and 108_3, the oxidesemiconductor film 108_2 can be distanced away from defect states.

In addition, the defect states might be more distant from the vacuumlevel than the conduction band minimum (E_(C)) of the oxidesemiconductor film 108_2 functioning as a channel region, so thatelectrons are likely to be accumulated in the defect states. When theelectrons are accumulated in the defect states, the electrons becomenegative fixed electric charge, so that the threshold voltage of thetransistor is shifted in the positive direction. Therefore, it ispreferable that the defect states be closer to the vacuum level than theconduction band minimum (E_(C)) of the oxide semiconductor film 108_2.Such a structure inhibits accumulation of electrons in the defectstates. As a result, the on-state current and the field-effect mobilityof the transistor can be increased.

The conduction band minimum of each of the oxide semiconductor films108_1 and 108_3 is closer to the vacuum level than that of the oxidesemiconductor film 108_2. A typical difference between the conductionband minimum of the oxide semiconductor film 108_2 and the conductionband minimum of each of the oxide semiconductor films 108_1 and 108_3 is0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. Thatis, the electron affinity of the oxide semiconductor film 108_2 ishigher than those of the oxide semiconductor films 108_1 and 108_3. Thedifference between the electron affinity of each of the oxidesemiconductor films 108_1 and 108_3 and the electron affinity of theoxide semiconductor film 108_2 is 0.15 eV or more or 0.5 eV or more and2 eV or less or 1 eV or less.

In such a structure, the oxide semiconductor film 108_2 serves as a mainpath of current. In other words, the oxide semiconductor film 108_2serves as a channel region, and the oxide semiconductor films 108_1 and108_3 serve as oxide insulating films. It is preferable that the oxidesemiconductor films 108_1 and 108_3 each include one or more metalelements constituting the oxide semiconductor film 108_2 in which achannel region is formed. With such a structure, interface scatteringhardly occurs at the interface between the oxide semiconductor film108_1 and the oxide semiconductor film 108_2 or at the interface betweenthe oxide semiconductor film 108_2 and the oxide semiconductor film108_3. Thus, the transistor can have high field-effect mobility becausethe movement of carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 108_1 and 108_3 fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 108_1 and108_3. Thus, the oxide semiconductor films 108_1 and 108_3 can bereferred to as oxide insulating films for such properties and/orfunctions. A material used for the oxide semiconductor films 108_1 and108_3 has a smaller electron affinity (a difference between the vacuumlevel and the conduction band minimum) than the oxide semiconductor film108_2 and is selected such that a difference (band offset) existsbetween the conduction band minimum of each of the oxide semiconductorfilms 108_1 and 108_3 and that of the oxide semiconductor film 108_2.Furthermore, to inhibit generation of a difference in threshold voltagedue to the value of the drain voltage, it is preferable to form theoxide semiconductor films 108_1 and 108_3 using a material whoseconduction band minimum is closer to the vacuum level than that of theoxide semiconductor film 108_2. For example, a difference between theconduction band minimum of the oxide semiconductor film 108_2 and theconduction band minimum of each of the oxide semiconductor films 108_1and 108_3 is preferably greater than or equal to 0.2 eV, furtherpreferably greater than or equal to 0.5 eV.

It is preferable that the oxide semiconductor films 108_1 and 108_3 nothave a spinel crystal structure. This is because if the oxidesemiconductor films 108_1 and 108_3 have a spinel crystal structure,constituent elements of the conductive films 120 a and 120 b might bediffused into the oxide semiconductor film 108_2 at the interfacebetween the spinel crystal structure and another region. Note that eachof the oxide semiconductor films 108_1 and 108_3 is preferably a CAAC-OSfilm described later, in which case a higher blocking property againstconstituent elements of the conductive films 120 a and 120 b, forexample, copper elements, can be obtained.

Although the example where an oxide semiconductor film formed using ametal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2, isused as each of the oxide semiconductor films 108_1 and 108_3 isdescribed in this embodiment, one embodiment of the present invention isnot limited thereto. For example, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:1:1, 1:1:1.2, 1:3:4, 1:3:6, 1:4:5, 1:5:6, or 1:10:1 may be used aseach of the oxide semiconductor films 108_1 and 108_3. Alternatively,oxide semiconductor films formed using a metal oxide target whose atomicratio of Ga to Zn is 10:1 may be used as the oxide semiconductor films108_1 and 108_3. In that case, it is suitable that an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:1:1 is used as the oxide semiconductor film108_2 and an oxide semiconductor film formed using a metal oxide targetwhose atomic ratio of Ga to Zn is 10:1 is used as each of the oxidesemiconductor films 108_1 and 108_3 because the difference between theconduction band minimum of the oxide semiconductor film 108_2 and theconduction band minimum of the oxide semiconductor film 108_1 or 108_3can be 0.6 eV or more.

When the oxide semiconductor films 108_1 and 108_3 are formed using ametal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, theatomic ratio of In to Ga and Zn in the oxide semiconductor films 108_1and 108_3 might be 1:β1:β2 (0<β1≤2, 0<β2≤2). When the oxidesemiconductor films 108_1 and 108_3 are formed using a metal oxidetarget whose atomic ratio of In to Ga and Zn is 1:3:4, the atomic ratioof In to Ga and Zn in the oxide semiconductor films 108_1 and 108_3might be 1:β3:β4 (1≤β3≤5, 2≤β4≤6). When the oxide semiconductor films108_1 and 108_3 are formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:3:6, the atomic ratio of In to Ga and Zn in theoxide semiconductor films 108_1 and 108_3 might be 1:β5:β6 (1≤β5≤5,4≤β6≤8).

<2-3. Structure Example 4 of Transistor>

FIG. 45A is a top view of a transistor 300A. FIG. 45B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.45A. FIG. 45C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 45A. Note that in FIG. 45A, some components of thetransistor 300A (e.g., an insulating film functioning as a gateinsulating film) are not illustrated to avoid complexity. The directionof the dashed-dotted line X1-X2 may be referred to as a channel lengthdirection, and the direction of the dashed-dotted line Y1-Y2 may bereferred to as a channel width direction. As in FIG. 45A, somecomponents are not illustrated in some cases in top views of transistorsdescribed below.

The transistor 300A illustrated in FIGS. 45A to 45C includes aconductive film 304 over a substrate 302, an insulating film 306 overthe substrate 302 and the conductive film 304, an insulating film 307over the insulating film 306, an oxide semiconductor film 308 over theinsulating film 307, a conductive film 312 a over the oxidesemiconductor film 308, and a conductive film 312 b over the oxidesemiconductor film 308. Over the transistor 300A, specifically, over theconductive films 312 a and 312 b and the oxide semiconductor film 308,an insulating film 314, an insulating film 316, and an insulating film318 are provided.

In the transistor 300A, the insulating films 306 and 307 function as thegate insulating films of the transistor 300A, and the insulating films314, 316, and 318 function as protective insulating films of thetransistor 300A. Furthermore, in the transistor 300A, the conductivefilm 304 functions as a gate electrode, the conductive film 312 afunctions as a source electrode, and the conductive film 312 b functionsas a drain electrode.

In this specification and the like, the insulating films 306 and 307 maybe referred to as a first insulating film, the insulating films 314 and316 may be referred to as a second insulating film, and the insulatingfilm 318 may be referred to as a third insulating film.

The transistor 300A illustrated in FIGS. 45A to 45C is a channel-etchedtransistor. The oxide semiconductor film of one embodiment of thepresent invention is suitable for a channel-etched transistor.

<2-4. Structure Example 5 of Transistor>

FIG. 46A is a top view of a transistor 300B. FIG. 46B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.46A. FIG. 46C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 46A.

The transistor 300B illustrated in FIGS. 46A to 46C includes theconductive film 304 over the substrate 302, the insulating film 306 overthe substrate 302 and the conductive film 304, the insulating film 307over the insulating film 306, the oxide semiconductor film 308 over theinsulating film 307, the insulating film 314 over the oxidesemiconductor film 308, the insulating film 316 over the insulating film314, the conductive film 312 a electrically connected to the oxidesemiconductor film 308 through an opening 341 a provided in theinsulating films 314 and 316, and the conductive film 312 b electricallyconnected to the oxide semiconductor film 308 through an opening 341 bprovided in the insulating films 314 and 316. Over the transistor 300B,specifically, over the conductive films 312 a and 312 b and theinsulating film 316, the insulating film 318 is provided.

In the transistor 300B, the insulating films 306 and 307 each functionas a gate insulating film of the transistor 300B, the insulating films314 and 316 each function as a protective insulating film of the oxidesemiconductor film 308, and the insulating film 318 functions as aprotective insulating film of the transistor 300B. Moreover, in thetransistor 300B, the conductive film 304 functions as a gate electrode,the conductive film 312 a functions as a source electrode, and theconductive film 312 b functions as a drain electrode.

The transistor 300A illustrated in FIGS. 45A to 45C has a channel-etchedstructure, whereas the transistor 300B in FIGS. 46A to 46C has achannel-protective structure. The oxide semiconductor film of oneembodiment of the present invention is suitable for a channel-protectivetransistor as well.

<2-5. Structure Example 6 of Transistor>

FIG. 47A is a top view of a transistor 300C. FIG. 47B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.47A. FIG. 47C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 47A.

The transistor 300C illustrated in FIGS. 47A to 47C is different fromthe transistor 300B in FIGS. 46A to 46C in the shapes of the insulatingfilms 314 and 316. Specifically, the insulating films 314 and 316 of thetransistor 300C have island shapes and are provided over a channelregion of the oxide semiconductor film 308. Other components are similarto those of the transistor 300B.

<2-6. Structure Example 7 of Transistor>

FIG. 48A is a top view of a transistor 300D. FIG. 48B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.48A. FIG. 48C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 48A.

The transistor 300D illustrated in FIGS. 48A to 48C includes theconductive film 304 over the substrate 302, the insulating film 306 overthe substrate 302 and the conductive film 304, the insulating film 307over the insulating film 306, the oxide semiconductor film 308 over theinsulating film 307, the conductive film 312 a over the oxidesemiconductor film 308, the conductive film 312 b over the oxidesemiconductor film 308, the insulating film 314 over the oxidesemiconductor film 308 and the conductive films 312 a and 312 b, theinsulating film 316 over the insulating film 314, the insulating film318 over the insulating film 316, and conductive films 320 a and 320 bover the insulating film 318.

In the transistor 300D, the insulating films 306 and 307 function asfirst gate insulating films of the transistor 300D, and the insulatingfilms 314, 316, and 318 function as second gate insulating films of thetransistor 300D. Furthermore, in the transistor 300D, the conductivefilm 304 functions as a first gate electrode, the conductive film 320 afunctions as a second gate electrode, and the conductive film 320 bfunctions as a pixel electrode used for a display device. The conductivefilm 312 a functions as a source electrode, and the conductive film 312b functions as a drain electrode.

As illustrated in FIG. 48C, the conductive film 320 a is connected tothe conductive film 304 in an opening portion 342 b and an openingportion 342 c provided in the insulating films 306, 307, 314, 316, and318. Thus, the same potential is applied to the conductive film 320 aand the conductive film 304.

The structure of the transistor 300D is not limited to that describedabove, in which the opening portions 342 b and 342 c are provided sothat the conductive film 320 a is connected to the conductive film 304.For example, a structure in which only one of the opening portions 342 band 342 c is provided so that the conductive film 320 a is connected tothe conductive film 304, or a structure in which the conductive film 320a is not connected to the conductive film 304 without providing theopening portions 342 b and 342 c may be employed. Note that in the casewhere the conductive film 320 a is not connected to the conductive film304, it is possible to apply different potentials to the conductive film320 a and the conductive film 304.

The conductive film 320 b is connected to the conductive film 312 bthrough an opening portion 342 a provided in the insulating films 314,316, and 318.

Note that the transistor 300D has the S-channel structure describedabove.

<2-7. Structure Example 8 of Transistor>

The oxide semiconductor film 308 included in the transistor 300A inFIGS. 45A to 45C may have a stacked-layer structure. FIGS. 49A and 49Band FIGS. 50A and 50B illustrate examples of such a case.

FIGS. 49A and 49B are cross-sectional views of a transistor 300E andFIGS. 50A and 50B are cross-sectional views of a transistor 300F. Thetop views of the transistors 300E and 300F are similar to that of thetransistor 300A illustrated in FIG. 45A.

The oxide semiconductor film 308 of the transistor 300E illustrated inFIGS. 49A and 49B includes an oxide semiconductor film 3081, an oxidesemiconductor film 308_2, and an oxide semiconductor film 308_3. Theoxide semiconductor film 308 of the transistor 300F illustrated in FIGS.50A and 50B includes the oxide semiconductor film 308_2 and the oxidesemiconductor film 308_3.

Note that the conductive film 304, the insulating film 306, theinsulating film 307, the oxide semiconductor film 308, the oxidesemiconductor film 308_1, the oxide semiconductor film 308_2, the oxidesemiconductor film 308_3, the conductive films 312 a and 312 b, theinsulating film 314, the insulating film 316, the insulating film 318,and the conductive films 320 a and 320 b can be formed using thematerials of the conductive film 106, the insulating film 116, the oxidesemiconductor film 108, the oxide semiconductor film 1081, the oxidesemiconductor film 1082, the oxide semiconductor film 1083, theconductive films 120 a and 120 b, the insulating film 104, theinsulating film 118, the insulating film 116, and the conductive film112, respectively, described above.

<2-8. Structure Example 9 of Transistor>

FIG. 51A is a top view of a transistor 300G. FIG. 51B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.51A. FIG. 51C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 51A.

The transistor 300G illustrated in FIGS. 51A to 51C includes theconductive film 304 over the substrate 302, the insulating film 306 overthe substrate 302 and the conductive film 304, the insulating film 307over the insulating film 306, the oxide semiconductor film 308 over theinsulating film 307, the conductive film 312 a over the oxidesemiconductor film 308, the conductive film 312 b over the oxidesemiconductor film 308, the insulating film 314 over the oxidesemiconductor film 308 and the conductive films 312 a and 312 b, theinsulating film 316 over the insulating film 314, the conductive film320 a over the insulating film 316, and the conductive film 320 b overthe insulating film 316.

The insulating films 306 and 307 have an opening 351. A conductive film312 c, which is electrically connected to the conductive film 304through the opening 351, is formed over the insulating films 306 and307. The insulating films 314 and 316 have an opening 352 a that reachesthe conductive film 312 b and an opening 352 b that reaches theconductive film 312 c.

The oxide semiconductor film 308 includes the oxide semiconductor film308_2 on the conductive film 304 side and the oxide semiconductor film308_3 over the oxide semiconductor film 308_2.

The insulating film 318 is provided over the transistor 300G. Theinsulating film 318 is formed to cover the insulating film 316, theconductive film 320 a, and the conductive film 320 b.

In the transistor 300G, the insulating films 306 and 307 function asfirst gate insulating films of the transistor 300G, and the insulatingfilms 314 and 316 function as second gate insulating films of thetransistor 300G, and the insulating film 318 functions as a protectiveinsulating film of the transistor 300G. Furthermore, in the transistor300G, the conductive film 304 functions as a first gate electrode, theconductive film 320 a functions as a second gate electrode, and theconductive film 320 b functions as a pixel electrode used for a displaydevice. Moreover, in the transistor 300G, the conductive film 312 afunctions as a source electrode, the conductive film 312 b functions asa drain electrode, and the conductive film 312 c functions as aconnection electrode.

Note that the transistor 300G has the S-channel structure describedabove.

The structures of the transistors 300A to 300G can be freely combinedwith each other.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 3

In this embodiment, examples of a display device that includes thesemiconductor device described in the above embodiments are describedbelow with reference to FIG. 52 to FIG. 59.

FIG. 52 is a top view illustrating an example of a display device. Adisplay device 700 in FIG. 52 includes a pixel portion 702 provided overa first substrate 701, a source driver circuit portion 704 and a gatedriver circuit portion 706 that are provided over the first substrate701, a sealant 712 provided to surround the pixel portion 702, thesource driver circuit portion 704, and the gate driver circuit portion706, and a second substrate 705 provided to face the first substrate701. The first substrate 701 and the second substrate 705 are sealedwith the sealant 712. That is, the pixel portion 702, the source drivercircuit portion 704, and the gate driver circuit portion 706 areenclosed by the first substrate 701, the sealant 712, and the secondsubstrate 705. Although not illustrated in FIG. 52, a display element isprovided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 that is electrically connected to the pixel portion 702, thesource driver circuit portion 704, and the gate driver circuit portion706 is provided in a region different from the region that is over thefirst substrate 701 and surrounded by the sealant 712. Furthermore, anFPC 716 is connected to the FPC terminal portion 708, and a variety ofsignals and the like are supplied from the FPC 716 to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706. Furthermore, a signal line 710 is connected to the pixelportion 702, the source driver circuit portion 704, the gate drivercircuit portion 706, and the FPC terminal portion 708. Through thesignal line 710, a variety of signals and the like are supplied from theFPC 716 to the pixel portion 702, the source driver circuit portion 704,the gate driver circuit portion 706, and the FPC terminal portion 708.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. The structure of the display device 700 is notlimited to the example shown here, in which the source driver circuitportion 704 and the gate driver circuit portion 706 as well as the pixelportion 702 are formed over the first substrate 701. For example, onlythe gate driver circuit portion 706 may be formed over the firstsubstrate 701, or only the source driver circuit portion 704 may beformed over the first substrate 701. In this case, a substrate overwhich a source driver circuit, a gate driver circuit, or the like isformed (e.g., a driver circuit board formed using a single crystalsemiconductor film or a polycrystalline semiconductor film) may beformed on the first substrate 701. Note that there is no particularlimitation on the method for connecting the separately prepared drivercircuit board, and a chip on glass (COG) method, a wire bonding method,or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors.

The display device 700 can include a variety of elements. As examples ofthe elements, electroluminescent (EL) element (e.g., an EL elementcontaining organic and inorganic materials, an organic EL element, aninorganic EL element, or an LED), a light-emitting transistor element (atransistor that emits light depending on current), an electron emitter,a liquid crystal element, an electronic ink display, an electrophoreticelement, an electrowetting element, a plasma display panel (PDP), microelectro mechanical systems (MEMS) display (e.g., a grating light valve(GLV), a digital micromirror device (DMD), a digital micro shutter (DMS)element, or an interferometric modulator display (IMOD) element), apiezoelectric ceramic display, and the like can be given.

An example of a display device including an EL element is an EL display.Examples of a display device including an electron emitter include afield emission display (FED) and an SED-type flat panel display (SED:surface-conduction electron-emitter display). An example of a displaydevice including a liquid crystal element is a liquid crystal display (atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display). An example ofa display device including an electronic ink display or anelectrophoretic element is electronic paper. In a transflective liquidcrystal display or a reflective liquid crystal display, some or all ofpixel electrodes may function as reflective electrodes. For example,some or all of pixel electrodes may contain aluminum, silver, or thelike. In this case, a memory circuit such as an SRAM can be providedunder the reflective electrodes, leading to lower power consumption.

As a display system of the display device 700, a progressive system, aninterlace system, or the like can be employed. Furthermore, colorelements controlled in pixels at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of an R pixel,a G pixel, a B pixel, and a W (white) pixel may be used. Alternatively,a color element may be composed of two colors of R, G, and B as inPenTile layout. The two colors may differ depending on the colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Note that the size of a display regionmay differ between dots of color elements. One embodiment of thedisclosed invention is not limited to a color display device; thedisclosed invention can also be applied to a monochrome display device.

A coloring layer (also referred to as a color filter) may be used toobtain a full-color display device in which white light (W) is used fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp). For example, a red (R) coloring layer, agreen (G) coloring layer, a blue (B) coloring layer, and a yellow (Y)coloring layer can be combined as appropriate. With the use of thecoloring layer, high color reproducibility can be obtained as comparedwith the case without the coloring layer. Here, by providing a regionwith a coloring layer and a region without a coloring layer, white lightin the region without the coloring layer may be directly utilized fordisplay. By partly providing the region without a coloring layer, adecrease in the luminance of a bright image due to the coloring layercan be suppressed, and power consumption can be reduced by approximately20% to 30% in some cases. In the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light in their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption may be further reduced as compared with the case of using acoloring layer.

As a coloring system, any of the following systems may be used: theabove-described color filter system in which part of white light isconverted into red light, green light, and blue light through colorfilters; a three-color system in which red light, green light, and bluelight are used; and a color conversion system or a quantum dot system inwhich part of blue light is converted into red light or green light.

In this embodiment, a structure including a liquid crystal element as adisplay element and a structure including an EL element as a displayelement are described with reference to FIG. 53 to FIG. 55. FIG. 53 andFIG. 54 are each a cross-sectional view taken along dashed-dotted lineQ-R in FIG. 52 and illustrate the structure including a liquid crystalelement as a display element. FIG. 55 is a cross-sectional view takenalong dashed-dotted line Q-R in FIG. 52 and illustrates the structureincluding an EL element as a display element.

Portions common to FIG. 53 to FIG. 55 are described first, and then,different portions are described.

<3-1. Portions Common to Display Devices>

The display device 700 in FIG. 53 to FIG. 55 includes a lead wiringportion 711, the pixel portion 702, the source driver circuit portion704, and the FPC terminal portion 708. The lead wiring portion 711includes the signal line 710. The pixel portion 702 includes atransistor 750 and a capacitor 790. The source driver circuit portion704 includes a transistor 752.

The transistor 750 and the transistor 752 each have a structure similarto that of the transistor 100A described above. Note that the transistor750 and the transistor 752 may each have the structure of any of theother transistors described in the above embodiments.

The transistor used in this embodiment includes an oxide semiconductorfilm that is highly purified and in which formation of oxygen vacanciesis inhibited. The transistor can have a low off-state current.Accordingly, an electrical signal such as an image signal can be heldfor a long time, and a long writing interval can be set in an on state.Accordingly, the frequency of refresh operation can be reduced, whichsuppresses power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high-speed operation.For example, in a liquid crystal display device that includes such atransistor capable of high-speed operation, a switching transistor in apixel portion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, no additional semiconductor deviceformed using a silicon wafer or the like is needed as a driver circuit;therefore, the number of components of the semiconductor device can bereduced. In addition, by using the transistor capable of high-speedoperation in the pixel portion, a high-quality image can be provided.

The capacitor 790 includes a lower electrode and an upper electrode. Thelower electrode is formed through a step of processing a conductive filmto be a conductive film functioning as a first gate electrode of thetransistor 750. The upper electrode is formed through a step ofprocessing a conductive film to be a conductive film functioning assource and drain electrodes or a second gate electrode of the transistor750. Between the lower electrode and the upper electrode, an insulatingfilm formed through a step of forming an insulating film to be aninsulating film functioning as a first gate insulating film of thetransistor 750 and insulating films formed through a step of forminginsulating films to be insulating films functioning as protectiveinsulating films over the transistor 750 are provided. That is, thecapacitor 790 has a stacked-layer structure in which an insulating filmfunctioning as a dielectric film is positioned between the pair ofelectrodes.

In FIG. 53 to FIG. 55, a planarization insulating film 770 is providedover the transistor 750, the transistor 752, and the capacitor 790.

Although FIG. 53 to FIG. 55 each illustrate an example in which thetransistor 750 included in the pixel portion 702 and the transistor 752included in the source driver circuit portion 704 have the samestructure, one embodiment of the present invention is not limitedthereto. For example, the pixel portion 702 and the source drivercircuit portion 704 may include different transistors. Specifically, astructure in which a top-gate transistor is used in the pixel portion702 and a bottom-gate transistor is used in the source driver circuitportion 704, or a structure in which a bottom-gate transistor is used inthe pixel portion 702 and a top-gate transistor is used in the sourcedriver circuit portion 704 may be employed. Note that the term “sourcedriver circuit portion 704” can be replaced by the term “gate drivercircuit portion.”

The signal line 710 is formed through the same process as the conductivefilms functioning as source electrodes and drain electrodes of thetransistors 750 and 752. In the case where the signal line 710 is formedusing a material containing a copper element, signal delay or the likedue to wiring resistance is reduced, which enables display on a largescreen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed through the same process as theconductive films functioning as source electrodes and drain electrodesof the transistors 750 and 752. The connection electrode 760 iselectrically connected to a terminal included in the FPC 716 through theanisotropic conductive film 780.

For example, glass substrates can be used as the first substrate 701 andthe second substrate 705. As the first substrate 701 and the secondsubstrate 705, flexible substrates may also be used. An example of theflexible substrate is a plastic substrate.

A structure 778 is provided between the first substrate 701 and thesecond substrate 705. The structure 778 is a columnar spacer obtained byselective etching of an insulating film and is provided to control thedistance (cell gap) between the first substrate 701 and the secondsubstrate 705. Alternatively, a spherical spacer may also be used as thestructure 778.

Alight-blocking film 738 functioning as a black matrix, a coloring film736 functioning as a color filter, and an insulating film 734 in contactwith the light-blocking film 738 and the coloring film 736 are providedon the second substrate 705 side.

<3-2. Structure Example of Display Device Including Liquid CrystalElement>

The display device 700 in FIG. 53 includes a liquid crystal element 775.The liquid crystal element 775 includes a conductive film 772, aconductive film 774, and a liquid crystal layer 776. The conductive film774 is provided on the second substrate 705 side and functions as acounter electrode. The display device 700 in FIG. 53 can display animage in such a manner that transmission or non-transmission of light iscontrolled by the alignment state in the liquid crystal layer 776 thatis changed depending on the voltage applied between the conductive film772 and the conductive film 774.

The conductive film 772 is electrically connected to the conductive filmfunctioning as the source electrode or the drain electrode of thetransistor 750. The conductive film 772 is formed over the planarizationinsulating film 770 and functions as a pixel electrode, that is, oneelectrode of the display element.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used as the conductive film 772. Forexample, a material containing an element selected from indium (In),zinc (Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material containing aluminum orsilver is preferably used for the conductive film that reflects visiblelight.

In the case where a conductive film that reflects visible light is usedas the conductive film 772, the display device 700 is a reflectiveliquid crystal display device. In the case where a conductive film thattransmits visible light is used as the conductive film 772, the displaydevice 700 is a transmissive liquid crystal display device.

The method for driving the liquid crystal element can be changed bychanging the structure over the conductive film 772, an example of thiscase is illustrated in FIG. 54. The display device 700 illustrated inFIG. 54 is an example of employing a horizontal electric field mode(e.g., an FFS mode) as a driving mode of the liquid crystal element. Inthe structure illustrated in FIG. 54, an insulating film 773 is providedover the conductive film 772, and the conductive film 774 is providedover the insulating film 773. In such a structure, the conductive film774 functions as a common electrode, and an electric field generatedbetween the conductive film 772 and the conductive film 774 through theinsulating film 773 can control the alignment state in the liquidcrystal layer 776.

Although not illustrated in FIG. 53 and FIG. 54, the conductive film 772and/or the conductive film 774 may be provided with an alignment film ona side in contact with the liquid crystal layer 776. Although notillustrated in FIG. 53 and FIG. 54, an optical member (opticalsubstrate) or the like, such as a polarizing member, a retardationmember, or an anti-reflection member, may be provided as appropriate.For example, circular polarization may be obtained by using a polarizingsubstrate and a retardation substrate. In addition, a backlight, asidelight, or the like may be used as a light source.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. These liquid crystal materials exhibit acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

In the case where a horizontal electric field mode is employed, a liquidcrystal exhibiting a blue phase for which an alignment film isunnecessary may be used. The blue phase is one of liquid crystal phases,which is generated just before a cholesteric phase changes into anisotropic phase when the temperature of a cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition containing a liquid crystal exhibiting a blue phase and achiral material has a short response time and optical isotropy, whicheliminates the need for an alignment process. An alignment film does notneed to be provided, and thus, rubbing treatment is not necessary;accordingly, electrostatic discharge damage caused by the rubbingtreatment can be prevented, and defects and damage of a liquid crystaldisplay device in the manufacturing process can be reduced. Moreover,the liquid crystal material that exhibits a blue phase has small viewingangle dependence.

In the case where a liquid crystal element is used as a display element,a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringefield switching (FFS) mode, an axially symmetric aligned micro-cell(ASM) mode, an optical compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as avertical alignment (VA) mode transmissive liquid crystal display devicemay also be used. There are some examples of a vertical alignment mode;for example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, and an ASV mode, or the like can beemployed.

<3-3. Display Device Including Light-Emitting Element>

The display device 700 illustrated in FIG. 55 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film772, an EL layer 786, and a conductive film 788. The display device 700illustrated in FIG. 55 can display an image by utilizing light emissionfrom the EL layer 786 of the light-emitting element 782. Note that theEL layer 786 contains an organic compound or an inorganic compound suchas a quantum dot.

Examples of materials that can be used for an organic compound include afluorescent material and a phosphorescent material. Examples ofmaterials that can be used for a quantum dot include a colloidal quantumdot material, an alloyed quantum dot material, a core-shell quantum dotmaterial, and a core quantum dot material. A material containingelements belonging to Groups 12 and 16, elements belonging to Groups 13and 15, or elements belonging to Groups 14 and 16, may be used.Alternatively, a quantum dot material containing an element such ascadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P),indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), oraluminum (Al) may be used.

The above-described organic compound and the inorganic compound can bedeposited by a method such as an evaporation method (including a vacuumevaporation method), a droplet discharge method (also referred to as anink-jet method), a coating method, or a gravure printing method. A lowmolecular material, a middle molecular material (including an oligomerand a dendrimer), or a high molecular material may be included in the ELlayer 786.

Here, a method for forming the EL layer 786 by a droplet dischargemethod is described with reference to FIGS. 58A to 58D. FIGS. 58A to 58Dare cross-sectional views illustrating the method for forming the ELlayer 786.

First, the conductive film 772 is formed over the planarizationinsulating film 770, and an insulating film 730 is formed to cover partof the conductive film 772 (see FIG. 58A).

Then, a droplet 784 is discharged to an exposed portion of theconductive film 772, which is an opening of the insulating film 730,from a droplet discharge apparatus 783, so that a layer 785 containing acomposition is formed. The droplet 784 is a composition containing asolvent and is attached to the conductive film 772 (see FIG. 58B).

Note that the step of discharging the droplet 784 may be performed underreduced pressure.

Next, the solvent is removed from the layer 785 containing thecomposition, and the resulting layer is solidified to form the EL layer786 (see FIG. 58C).

The solvent may be removed by drying or heating.

Next, the conductive film 788 is formed over the EL layer 786; thus, thelight-emitting element 782 is formed (see FIG. 58D).

When the EL layer 786 is formed by a droplet discharge method asdescribed above, the composition can be selectively discharged;accordingly, waste of material can be reduced. Furthermore, alithography process or the like for shaping is not needed, and thus, theprocess can be simplified and cost reduction can be achieved.

The droplet discharge method described above is a general term for ameans including a nozzle equipped with a composition discharge openingor a means to discharge droplets such as a head having one or aplurality of nozzles.

Next, a droplet discharge apparatus used for the droplet dischargemethod is described with reference to FIG. 59. FIG. 59 is a conceptualdiagram illustrating a droplet discharge apparatus 1400.

The droplet discharge apparatus 1400 includes a droplet discharge means1403. In addition, the droplet discharge means 1403 is equipped with ahead 1405 and a head 1412.

The heads 1405 and 1412 are connected to a control means 1407, and thiscontrol means 1407 is controlled by a computer 1410; thus, apreprogrammed pattern can be drawn.

The drawing may be conducted at a timing, for example, based on a marker1411 formed over a substrate 1402. Alternatively, the reference pointmay be determined on the basis of an outer edge of the substrate 1402.Here, the marker 1411 is detected by an imaging means 1404 and convertedinto a digital signal by an image processing means 1409. Then, thedigital signal is recognized by the computer 1410, and then, a controlsignal is generated and transmitted to the control means 1407.

An image sensor or the like using a charge coupled device (CCD) or acomplementary metal-oxide-semiconductor (CMOS) can be used as theimaging means 1404. Note that information about a pattern to be formedover the substrate 1402 is stored in a storage medium 1408, and acontrol signal is transmitted to the control means 1407 based on theinformation, so that each of the heads 1405 and 1412 of the dropletdischarge means 1403 can be individually controlled. The heads 1405 and1412 are supplied with a material to be discharged from material supplysources 1413 and 1414 through pipes, respectively.

Inside the head 1405, a space as indicated by a dotted line 1406 to befilled with a liquid material and a nozzle which is a discharge outletare provided. Although it is not shown, an inside structure of the head1412 is similar to that of the head 1405. When the nozzle sizes of theheads 1405 and 1412 are different from each other, different materialswith different widths can be discharged simultaneously. Each head candischarge and draw a plurality of light emitting materials. In the caseof drawing over a large area, the same material can be simultaneouslydischarged to be drawn from a plurality of nozzles in order to improvethroughput. When a large substrate is used, the heads 1405 and 1412 canfreely scan the substrate in directions indicated by arrows X, Y, and Zin FIG. 59, and a region in which a pattern is drawn can be freely set.Thus, a plurality of the same patterns can be drawn over one substrate.

Furthermore, a step of discharging the composition may be performedunder reduced pressure. A substrate may be heated when the compositionis discharged. After discharging the composition, either drying orbaking or both of them are performed. Both the drying and baking areheat treatments but different in purpose, temperature, and time period.The steps of drying and baking are performed under normal pressure orunder reduced pressure by laser irradiation, rapid thermal annealing,heating using a heating furnace, or the like. Note that there is noparticular limitation on the timing of the heat treatment and the numberof times of the heat treatment. The temperature for performing each ofthe steps of drying and baking in a favorable manner depends on thematerials of the substrate and the properties of the composition.

In the above-described manner, the EL layer 786 can be formed with thedroplet discharge apparatus.

The display device 700 shown in FIG. 55 is described again.

In the display device 700 in FIG. 55, the insulating film 730 isprovided over the planarization insulating film 770 and the conductivefilm 772. The insulating film 730 covers part of the conductive film772. Note that the light-emitting element 782 has a top-emissionstructure. Thus, the conductive film 788 has a light-transmittingproperty and transmits light emitted from the EL layer 786. Although thetop-emission structure is described as an example in this embodiment,the structure is not limited thereto. For example, a bottom-emissionstructure in which light is emitted to the conductive film 772 side or adual-emission structure in which light is emitted to both the conductivefilm 772 side and the conductive film 788 side may also be employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided in the leadwiring portion 711 and the source driver circuit portion 704 to overlapwith the insulating film 730. The coloring film 736 and thelight-blocking film 738 are covered with the insulating film 734. Aspace between the light-emitting element 782 and the insulating film 734is filled with a sealing film 732. The structure of the display device700 is not limited to the example in FIG. 55, in which the coloring film736 is provided. For example, a structure without the coloring film 736may also be employed in the case where the EL layer 786 is formed byseparate coloring.

<3-4. Structure Example of Display Device Provided with Input/OutputDevice>

An input/output device may be provided in the display device 700illustrated in FIG. 54 and FIG. 55. As an example of the input/outputdevice, a touch panel or the like can be given.

FIG. 56 illustrates a structure in which the display device 700illustrated in FIG. 54 includes a touch panel 791. FIG. 57 illustrates astructure in which the display device 700 illustrated in FIG. 55includes the touch panel 791.

FIG. 56 is a cross-sectional view of the structure in which the touchpanel 791 is provided in the display device 700 illustrated in FIG. 54,and FIG. 57 is a cross-sectional view of the structure in which thetouch panel 791 is provided in the display device 700 illustrated inFIG. 55.

First, the touch panel 791 illustrated in FIG. 56 and FIG. 57 isdescribed below.

The touch panel 791 illustrated in FIG. 56 and FIG. 57 is what is calledan in-cell touch panel provided between the substrate 705 and thecoloring film 736. The touch panel 791 is formed on the substrate 705side before the coloring film 736 is formed.

Note that the touch panel 791 includes the light-blocking film 738, aninsulating film 792, an electrode 793, an electrode 794, an insulatingfilm 795, an electrode 796, and an insulating film 797. A change in themutual capacitance between the electrodes 793 and 794 can be detectedwhen an object such as a finger or a stylus approaches, for example.

A portion in which the electrode 793 intersects with the electrode 794is illustrated in the upper portion of the transistor 750 illustrated inFIG. 56 and FIG. 57. The electrode 796 is electrically connected to thetwo electrodes 793 between which the electrode 794 is sandwiched throughopenings provided in the insulating film 795. Note that a structure inwhich a region where the electrode 796 is provided is provided in thepixel portion 702 is illustrated in FIG. 56 and FIG. 57 as an example;however, one embodiment of the present invention is not limited thereto.For example, the region where the electrode 796 is provided may beprovided in the source driver circuit portion 704.

The electrode 793 and the electrode 794 are provided in a regionoverlapping with the light-blocking film 738. As illustrated in FIG. 56,it is preferable that the electrode 793 not overlap with thelight-emitting element 782. As illustrated in FIG. 57, it is preferablethat the electrode 793 not overlap with the liquid crystal element 775.In other words, the electrode 793 has an opening in a region overlappingwith the light-emitting element 782 and the liquid crystal element 775.That is, the electrode 793 has a mesh shape. With such a structure, theelectrode 793 does not block light emitted from the light-emittingelement 782, or alternatively the electrode 793 does not block lighttransmitted through the liquid crystal element 775. Thus, sinceluminance is hardly reduced even when the touch panel 791 is provided, adisplay device with high visibility and low power consumption can beobtained. Note that the electrode 794 can have a structure similar tothat of the electrode 793.

Since the electrode 793 and the electrode 794 do not overlap with thelight-emitting element 782, a metal material having low transmittancewith respect to visible light can be used for the electrode 793 and theelectrode 794. Furthermore, since the electrode 793 and the electrode794 do not overlap with the liquid crystal element 775, a metal materialhaving low transmittance with respect to visible light can be used forthe electrode 793 and the electrode 794.

Thus, as compared with the case of using an oxide material whosetransmittance of visible light is high, resistance of the electrodes 793and 794 can be reduced, whereby sensitivity of the sensor of the touchpanel can be increased.

For example, a conductive nanowire may be used for the electrodes 793,794, and 796. The nanowire may have a mean diameter of greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm, further preferablygreater than or equal to 5 nm and less than or equal to 25 nm. As thenanowire, a carbon nanotube or a metal nanowire such as an Ag nanowire,a Cu nanowire, or an Al nanowire may be used. For example, in the casewhere an Ag nanowire is used for any one of or all of electrodes 793,794, and 796, the transmittance of visible light can be greater than orequal to 89% and the sheet resistance can be greater than or equal to 40Ω/sq. and less than or equal to 100 Ω/sq.

Although the structure of the in-cell touch panel is illustrated in FIG.56 and FIG. 57, one embodiment of the present invention is not limitedthereto. For example, a touch panel formed over the display device 700,what is called an on-cell touch panel, or a touch panel attached to thedisplay device 700, what is called an out-cell touch panel may be used.

In this manner, the display device of one embodiment of the presentinvention can be combined with various types of touch panels.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

In this embodiment, a display device including a semiconductor device ofone embodiment of the present invention is described with reference toFIGS. 60A to 60C.

<4. Circuit Configuration of Display Device>

A display device illustrated in FIG. 60A includes a region includingpixels of display elements (hereinafter referred to as a pixel portion502), a circuit portion that is provided outside the pixel portion 502and includes a circuit for driving the pixels (hereinafter, the circuitportion is referred to as a driver circuit portion 504), circuits havinga function of protecting elements (hereinafter, the circuits arereferred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formedover a substrate over which the pixel portion 502 is formed. Thus, thenumber of components and the number of terminals can be reduced. Whenpart or the whole of the driver circuit portion 504 is not formed overthe substrate over which the pixel portion 502 is formed, the part orthe whole of the driver circuit portion 504 can be mounted by COG ortape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X (X is a natural number of 2 or more) rowsand Y (Y is a natural number of 2 or more) columns (hereinafter, thecircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter referred to as scan lines GL_1 to GL_X). Note that aplurality of gate drivers 504 a may be provided to control the scanlines GL_1 to GL_X separately. Alternatively, the gate driver 504 a hasa function of supplying an initialization signal. Without being limitedthereto, another signal can be supplied from the gate driver 504 a.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (image signal) from which a datasignal is generated, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 fromthe image signal. In addition, the source driver 504 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the source driver 504 b has a function of controlling thepotentials of wirings supplied with data signals (hereinafter referredto as data lines DL_1 to DL_Y). Alternatively, the source driver 504 bhas a function of supplying an initialization signal. Without beinglimited thereto, another signal can be supplied from the source driver504 b.

The source driver 504 b includes a plurality of analog switches, forexample. The source driver 504 b can output, as data signals,time-divided image signals obtained by sequentially turning on theplurality of analog switches. The source driver 504 b may include ashift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of X orless, and n is a natural number of Y or less), a pulse signal is inputfrom the gate driver 504 a through the scan line GL_m, and a data signalis input from the source driver 504 b through the data line DL_n inaccordance with the potential of the scan line GL_m.

The protection circuit 506 in FIG. 60A is connected to, for example, thescan line GL between the gate driver 504 a and the pixel circuit 501.Alternatively, the protection circuit 506 is connected to the data lineDL between the source driver 504 b and the pixel circuit 501.Alternatively, the protection circuit 506 can be connected to a wiringbetween the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 refers to a portion having terminals forinputting power, control signals, and image signals from externalcircuits to the display device.

The protection circuit 506 electrically connects a wiring connected tothe protection circuit to another wiring when a potential out of acertain range is supplied to the wiring connected to the protectioncircuit.

As illustrated in FIG. 60A, the protection circuits 506 provided for thepixel portion 502 and the driver circuit portion 504 can improve theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like. Note that the configurationof the protection circuits 506 is not limited thereto; for example, theprotection circuit 506 can be connected to the gate driver 504 a or thesource driver 504 b. Alternatively, the protection circuit 506 can beconnected to the terminal portion 507.

One embodiment of the present invention is not limited to the example inFIG. 60A, in which the driver circuit portion 504 includes the gatedriver 504 a and the source driver 504 b. For example, only the gatedriver 504 a may be formed, and a separately prepared substrate overwhich a source driver circuit is formed (e.g., a driver circuit boardformed using a single crystal semiconductor film or a polycrystallinesemiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 60A can have theconfiguration illustrated in FIG. 60B, for example.

The pixel circuit 501 in FIG. 60B includes a liquid crystal element 570,a transistor 550, and a capacitor 560. As the transistor 550, thetransistor described in the above embodiment can be used.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set as appropriate in accordance with the specificationsof the pixel circuit 501. The alignment state of the liquid crystalelement 570 depends on data written thereto. A common potential may besupplied to the one of the pair of electrodes of the liquid crystalelement 570 included in each of the plurality of pixel circuits 501. Thepotential supplied to the one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 may differ between rows.

Examples of a method for driving the display device including the liquidcrystal element 570 include a TN mode, an STN mode, a VA mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVAmode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFSmode, and a transverse bend alignment (TBA) mode. Other examples of themethod for driving the display device include an electrically controlledbirefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC)mode, a polymer network liquid crystal (PNLC) mode, and a guest-hostmode. Without being limited thereto, various liquid crystal elements anddriving methods can be used.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other of thesource electrode and the drain electrode of the transistor 550 iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 isconfigured to be turned on or off to control whether a data signal iswritten.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other of the pair ofelectrodes of the capacitor 560 is electrically connected to the otherof the pair of electrodes of the liquid crystal element 570. Thepotential of the potential supply line VL is set as appropriate inaccordance with the specifications of the pixel circuit 501. Thecapacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuits 501 inFIG. 60B, the gate driver 504 a in FIG. 60A sequentially selects thepixel circuits 501 row by row to turn on the transistors 550, and datasignals are written.

When the transistor 550 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 60Acan have the configuration illustrated in FIG. 60C, for example.

The pixel circuit 501 in FIG. 60C includes transistors 552 and 554, acapacitor 562, and a light-emitting element 572. The transistordescribed in the above embodiment can be used as the transistor 552and/or the transistor 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring through which a data signal issupplied (hereinafter referred to as a data line DL_n). A gate electrodeof the transistor 552 is electrically connected to a wiring throughwhich a gate signal is supplied (hereinafter referred to as a scan lineGL_m).

The transistor 552 is configured to be turned on or off to controlwhether a data signal is written.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other of the pairof electrodes of the capacitor 562 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. A gateelectrode of the transistor 554 is electrically connected to the otherof the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other ofthe anode and the cathode of the light-emitting element 572 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) can be used, for example.Note that the light-emitting element 572 is not limited thereto and maybe an inorganic EL element including an inorganic material.

A high power supply potential V_(DD) is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential V_(SS) is supplied to the other of the potential supplyline VL_a and the potential supply line VL_b.

In the display device including the pixel circuits 501 in FIG. 60C, thegate driver 504 a in FIG. 60A sequentially selects the pixel circuits501 row by row to turn on the transistors 552, and data signals arewritten.

When the transistor 552 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 5

In this embodiment, a display module and electronic devices, each ofwhich includes a semiconductor device of one embodiment of the presentinvention, are described with reference to FIG. 61, FIGS. 62A to 62E,FIGS. 63A to 63G, FIGS. 64A to 64E, FIGS. 65A and 65B, and FIGS. 66A and66B.

<5-1. Display Module>

In a display module 7000 illustrated in FIG. 61, a touch panel 7004connected to an FPC 7003, a display panel 7006 connected to an FPC 7005,a backlight 7007, a frame 7009, a printed-circuit board 7010, and abattery 7011 are provided between an upper cover 7001 and a lower cover7002.

The semiconductor device of one embodiment of the present invention canbe used for the display panel 7006, for example.

The shapes and sizes of the upper cover 7001 and the lower cover 7002can be changed as appropriate in accordance with the sizes of the touchpanel 7004 and the display panel 7006.

The touch panel 7004 can be a resistive touch panel or a capacitivetouch panel and overlap with the display panel 7006. Alternatively, acounter substrate (sealing substrate) of the display panel 7006 can havea touch panel function. Alternatively, a photosensor may be provided ineach pixel of the display panel 7006 to form an optical touch panel.

The backlight 7007 includes a light source 7008. One embodiment of thepresent invention is not limited to the structure in FIG. 61, in whichthe light source 7008 is provided over the backlight 7007. For example,a structure in which the light source 7008 is provided at an end portionof the backlight 7007 and a light diffusion plate is further providedmay be employed. Note that the backlight 7007 need not be provided inthe case where a self-luminous light-emitting element such as an organicEL element is used or in the case where a reflective panel or the likeis employed.

The frame 7009 protects the display panel 7006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed-circuit board 7010. The frame 7009 may alsofunction as a radiator plate.

The printed-circuit board 7010 includes a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or the separate battery7011 may be used. The battery 7011 can be omitted in the case where acommercial power source is used.

The display module 7000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<5-2. Electronic Device 1>

Next, FIGS. 62A to 62E illustrate examples of electronic devices.

FIG. 62A is an external view of a camera 8000 to which a finder 8100 isattached.

The camera 8000 includes a housing 8001, a display portion 8002,operation buttons 8003, a shutter button 8004, and the like.Furthermore, an attachable lens 8006 is attached to the camera 8000.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in thehousing 8001.

Images can be taken with the camera 8000 at the press of the shutterbutton 8004. In addition, images can be taken at the touch of thedisplay portion 8002 that serves as a touch panel.

The housing 8001 of the camera 8000 includes a mount including anelectrode, so that the finder 8100, a stroboscope, or the like can beconnected to the housing 8001.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a mount for engagement with the mount of thecamera 8000 so that the finder 8100 can be connected to the camera 8000.The mount includes an electrode, and an image or the like received fromthe camera 8000 through the electrode can be displayed on the displayportion 8102.

The button 8103 serves as a power button. The display portion 8102 canbe turned on and off with the button 8103.

A display device of one embodiment of the present invention can be usedin the display portion 8002 of the camera 8000 and the display portion8102 of the finder 8100.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIG. 62A, the housing 8001 of the camera 8000 mayinclude a finder having a display device.

FIG. 62B is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. The movement of the eyeball and the eyelid of auser is captured by a camera in the main body 8203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as tobe in contact with the user. The main body 8203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 8203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 8201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The display device of one embodiment of the present invention can beused in the display portion 8204.

FIGS. 62C to 62E are external views of a head-mounted display 8300. Thehead-mounted display 8300 includes a housing 8301, a display portion8302, an object for fixing, such as a band, 8304, and a pair of lenses8305.

A user can see display on the display portion 8302 through the lenses8305. It is favorable that the display portion 8302 be curved. When thedisplay portion 8302 is curved, a user can feel high realistic sensationof images. Although the structure described in this embodiment as anexample has one display portion 8302, the number of the display portions8302 provided is not limited to one. For example, two display portions8302 may be provided, in which case one display portion is provided forone corresponding user's eye, so that three-dimensional display usingparallax or the like is possible.

The display device of one embodiment of the present invention can beused in the display portion 8302. The display device including thesemiconductor device of one embodiment of the present invention has anextremely high resolution; thus, even when an image is magnified usingthe lenses 8305 as illustrated in FIG. 62E, the user does not perceivepixels, and thus a more realistic image can be displayed.

<5-3. Electronic Device 2>

Next, FIGS. 63A to 63G and FIGS. 64A to 64E illustrate examples ofelectronic devices that are different from those illustrated in FIGS.62A to 62E.

Electronic devices illustrated in FIGS. 63A to 63G include a housing9000, a display portion 9001, a speaker 9003, an operation key 9005(including a power switch or an operation switch), a connection terminal9006, a sensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices in FIGS. 63A to 63G and FIGS. 64A to 64E have avariety of functions such as a function of displaying a variety ofinformation (e.g., a still image, a moving image, and a text image) onthe display portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling processingwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading out a program or datastored in a memory medium and displaying it on the display portion. Notethat functions of the electronic devices in FIGS. 63A to 63G and FIGS.64A to 64E are not limited thereto, and the electronic devices can havea variety of functions. Although not illustrated in FIGS. 63A to 63G andFIGS. 64A to 64E, the electronic devices may each have a plurality ofdisplay portions. Furthermore, the electronic devices may each beprovided with a camera and the like to have a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices in FIGS. 63A to 63G and FIGS. 64A to 64E aredescribed in detail below.

FIG. 63A is a perspective view illustrating a television device 9100.The television device 9100 can include the display portion 9001 having alarge screen size of, for example, 50 inches or more, or 100 inches ormore.

FIG. 63B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal 9101 can be used as asmartphone. Note that the portable information terminal 9101 may includea speaker, a connection terminal, a sensor, or the like. The portableinformation terminal 9101 can display text and image information on itsplurality of surfaces. For example, three operation buttons 9050 (alsoreferred to as operation icons or simply as icons) can be displayed onone surface of the display portion 9001. Furthermore, information 9051indicated by dashed rectangles can be displayed on another surface ofthe display portion 9001. Examples of the information 9051 includedisplay indicating reception of an e-mail, a social networking service(SNS) message, or a telephone call, the title and sender of an e-mail oran SNS message, date, time, remaining battery, and reception strength ofan antenna. Alternatively, the operation buttons 9050 or the like may bedisplayed in place of the information 9051.

FIG. 63C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) on theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. The user can see the display withouttaking out the portable information terminal 9102 from the pocket anddecide whether to answer the call.

FIG. 63D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, reading and editing texts, music reproduction, Internetcommunication, and a computer game. The display surface of the displayportion 9001 is curved, and display can be performed on the curveddisplay surface. The portable information terminal 9200 can employ nearfield communication conformable to a communication standard. Forexample, hands-free calling can be achieved by mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication. Moreover, the portable information terminal 9200includes the connection terminal 9006 and can perform direct datacommunication with another information terminal via a connector.Charging through the connection terminal 9006 is also possible. Notethat the charging operation may be performed by wireless power feedingwithout using the connection terminal 9006.

FIGS. 63E, 63F, and 63G are perspective views of a foldable portableinformation terminal 9201 that is opened, that is shifted from theopened state to the folded state or from the folded state to the openedstate, and that is folded, respectively. The portable informationterminal 9201 is highly portable when folded. When the portableinformation terminal 9201 is opened, a seamless large display region ishighly browsable. The display portion 9001 of the portable informationterminal 9201 is supported by three housings 9000 joined by hinges 9055.By being folded at the hinges 9055 between the two adjacent housings9000, the portable information terminal 9201 can be reversibly changedin shape from the opened state to the folded state. For example, theportable information terminal 9201 can be bent with a radius ofcurvature greater than or equal to 1 mm and less than or equal to 150mm.

FIG. 64A is a perspective view of a television device 9150 that isdifferent from the television device shown in FIG. 63A. In thetelevision device 9150, a display portion 9152 is incorporated in ahousing 9151. Here, the housing 9151 is supported by a stand 9153. Inthe television device 9150, the display portion 9152 and the housing9151 are formed unlike in the television device 9100.

The television device 9150 illustrated in FIG. 64A can be operated withan operation switch of the housing 9151 or a separate remote controller9154. The display portion 9152 may include a touch sensor, and can beoperated by touch on the display portion 9152 with a finger or the like.The remote controller 9154 may be provided with a display portion fordisplaying data output from the remote controller 9154. With operationkeys or a touch panel of the remote controller 9154, channels and volumecan be controlled and images displayed on the display portion 9152 canbe controlled.

Note that the television device 9150 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 64B is a perspective view illustrating a notebook personal computer9250. The notebook personal computer 9250 includes a housing 9251, adisplay portion 9252, a keyboard 9253, a pointing device 9254, and thelike.

FIG. 64C is a perspective view showing a slot machine 9300 that is astationary game machine. In the slot machine 9300, a display portion9303 is incorporated in a housing 9301. In addition, the slot machine9300 includes an operation means such as a start lever 9304 or a stopswitch 9305, a coin slot, a speaker, a light source 9306 for a sensor, asensor 9302, and the like.

FIG. 64D is an external view of an automobile 9400. FIG. 64E illustratesa driver's seat of the automobile 9400. The automobile 9400 includes acar body 9401, wheels 9402, a windshield 9403, lights 9404, fog lamps9405, and the like.

The display device of one embodiment of the present invention can beused in a display portion or the like of the automobile 9400. Forexample, the display device of one embodiment of the present inventioncan be used in display portions 9410 to 9417 illustrated in FIG. 64E.

The display portion 9410 and the display portion 9411 are provided inthe automobile windshield. The display device of one embodiment of thepresent invention can be a see-through device, through which theopposite side can be seen, by using a light-transmitting conductivematerial for its electrodes. Such a see-through display device does nothinder driver's vision during the driving of the automobile 9400.Therefore, the display device of one embodiment of the present inventioncan be provided in the windshield of the automobile 9400. Note that inthe case where a transistor or the like is provided in the displaydevice, a transistor having light-transmitting properties, such as anorganic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

The display portion 9412 is provided on a pillar portion. The displayportion 9413 is provided on a dashboard. For example, an image taken byan imaging unit provided in the car body is displayed on the displayportion 9412, whereby the view hindered by the pillar portion can becompensated. Similarly, the display portion 9413 can compensate for theview hindered by the dashboard and the display portion 9414 cancompensate for the view hindered by the door. That is, showing an imagetaken by an imaging unit provided on the outside of the car body leadsto elimination of blind areas and enhancement of safety. In addition,showing an image so as to compensate for the area which a driver cannotsee makes it possible for the driver to confirm safety easily andcomfortably.

The display portion 9417 is provided in a steering wheel. The displayportion 9415, the display portion 9416, or the display portion 9417 canprovide a variety of kinds of information such as navigation data, aspeedometer, a tachometer, a mileage, a fuel meter, a gearshiftindicator, and air-condition setting. The content, layout, or the likeof the display on the display portions can be changed freely by a useras appropriate. The information listed above can also be displayed onthe display portions 9410 to 9414.

The display portions 9410 to 9417 can also be used as lighting devices.

FIG. 65A is a perspective view illustrating a digital signage 9600. Thedigital signage 9600 can include a display portion 9601, a housing 9602,and a speaker 9603. As shown in FIG. 65B, the digital signage 9600 maybe mounted on a cylindrical pillar.

Next, an example of an electronic device that is different from theelectronic devices illustrated in FIGS. 62A to 62E, FIGS. 63A to 63G,and FIGS. 64A to 64E is illustrated in FIGS. 66A and 66B. FIGS. 66A and66B are perspective views of a display device including a plurality ofdisplay panels. Note that the plurality of display panels are wound inthe perspective view in FIG. 66A, and are unwound in the perspectiveview in FIG. 66B.

A display device 9500 illustrated in FIGS. 66A and 66B includes aplurality of display panels 9501, a hinge 9511, and a bearing 9512. Theplurality of display panels 9501 each include a display region 9502 anda light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacentdisplay panels 9501 are provided so as to partly overlap with eachother. For example, the light-transmitting regions 9503 of the twoadjacent display panels 9501 can be overlapped each other. A displaydevice having a large screen can be obtained with the plurality ofdisplay panels 9501. The display device is highly versatile because thedisplay panels 9501 can be wound depending on its use.

The display device shown in FIGS. 66A and 66B can easily have a largescreen and thus be used also as the above digital signage.

Moreover, although the display regions 9502 of the adjacent displaypanels 9501 are separated from each other in FIGS. 66A and 66B, withoutlimitation to this structure, the display regions 9502 of the adjacentdisplay panels 9501 may overlap with each other without any space sothat a continuous display region 9502 is obtained, for example.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic device that does not have a display portion.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 6

In this embodiment, one mode of a semiconductor device is described withreference to FIGS. 67A and 67B, FIG. 68, and FIG. 69.

Structure Examples

Examples of a semiconductor device (memory device) including a capacitorof one embodiment of the present invention are illustrated in FIGS. 67Aand 67B, FIG. 68, and FIG. 69. Note that FIG. 67A is a circuit diagramcorresponding to each of FIG. 68 and FIG. 69.

<Circuit Configuration of Semiconductor Device>

Semiconductor devices illustrated in FIG. 67A, FIG. 68, and FIG. 69 eachinclude a transistor 3300, a transistor 3200, and a capacitor 3100.

The transistor 3200 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 3200 is small, by using thetransistor 3200 in a semiconductor device (memory device), stored datacan be retained for a long time. In other words, it is possible toobtain a semiconductor device (memory device) which does not requirerefresh operation or has an extremely low frequency of the refreshoperation, which leads to a sufficient reduction in power consumption.

In FIG. 67A, a first wiring 3001 is electrically connected to a sourceof the transistor 3300. A second wiring 3002 is electrically connectedto a drain of the transistor 3300. A third wiring 3003 is electricallyconnected to one of a source and a drain of the transistor 3200. Afourth wiring 3004 is electrically connected to a gate of the transistor3200. A gate of the transistor 3300 and the other of the source and thedrain of the transistor 3200 are electrically connected to one electrodeof the capacitor 3100. A fifth wiring 3005 is electrically connected tothe other electrode of the capacitor 3100.

The semiconductor device in FIG. 67A has a feature that the potential ofthe gate of the transistor 3300 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and retaining of data will be described. First, the potential ofthe fourth wiring 3004 is set to a potential at which the transistor3200 is turned on, so that the transistor 3200 is turned on.Accordingly, the potential of the third wiring 3003 is supplied to anode FG where the gate of the transistor 3300 and the one electrode ofthe capacitor 3100 are electrically connected to each other. That is, apredetermined charge is supplied to the gate of the transistor 3300(writing). Here, one of two kinds of charges providing differentpotential levels (hereinafter referred to as a low-level charge and ahigh-level charge) is supplied. After that, the potential of the fourthwiring 3004 is set to a potential at which the transistor 3200 is turnedoff, so that the transistor 3200 is turned off. Thus, the charge isretained at the node FG (retaining).

In the case where the off-state current of the transistor 3200 is low,the electric charge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the node FG. This is because in the case ofusing an n-channel transistor as the transistor 3300, an apparentthreshold voltage V_(th_H) at the time when the high-level electriccharge is given to the gate of the transistor 3300 is lower than anapparent threshold voltage V_(th_L) at the time when the low-levelelectric charge is given to the gate of the transistor 3300. Here, anapparent threshold voltage refers to the potential of the fifth wiring3005 which is needed to make the transistor 3300 be in an on state.Thus, the potential of the fifth wiring 3005 is set to a potential V₀which is between V_(th_H) and V_(th_L), whereby charge supplied to thenode FG can be determined. For example, in the case where the high-levelcharge is supplied to the node FG in writing and the potential of thefifth wiring 3005 is V₀ (>V_(th) H), the transistor 3300 is brought intoan on state. On the other hand, in the case where the low-level chargeis supplied to the node FG in writing, even when the potential of thefifth wiring 3005 is V₀ (<V_(th_L)), the transistor 3300 remains in theoff state. Thus, the data retained in the node FG can be read bydetermining the potential of the second wiring 3002.

By arranging semiconductor devices each having the structure illustratedin FIG. 67A in a matrix, a memory device (memory cell array) can beformed.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell is read in read operation. Aconfiguration in which only data of a desired memory cell can be read bysupplying a potential at which the transistor 3300 is turned offregardless of the charge supplied to the node FG, that is, a potentiallower than V_(th_H) is supplied to the fifth wiring 3005 of memory cellsfrom which data is not read may be employed. Alternatively, aconfiguration in which only data of a desired memory cell can be read bysupplying a potential at which the transistor 3300 is turned onregardless of the charge supplied to the node FG, that is, a potentialhigher than V_(th_L) is supplied to the fifth wiring 3005 of memorycells from which data is not read may be employed.

<Circuit Configuration 2 of Semiconductor Device>

A semiconductor device in FIG. 67B is different from the semiconductordevice in FIG. 67A in that the transistor 3300 is not provided. Also inthis case, data can be written and retained in a manner similar to thatof the semiconductor device in FIG. 67A.

Reading of data in the semiconductor device in FIG. 67B is described.When the transistor 3200 is brought into an on state, the third wiring3003 which is in a floating state and the capacitor 3100 are broughtinto conduction, and the electric charge is redistributed between thethird wiring 3003 and the capacitor 3100. As a result, the potential ofthe third wiring 3003 changes. The amount of change in the potential ofthe third wiring 3003 varies depending on the potential of the oneelectrode of the capacitor 3100 (or the electric charge accumulated inthe capacitor 3100).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 3100, C is the capacitance of thecapacitor 3100, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of the oneelectrode of the capacitor 3100 is V₁ and V₀(V₁>V₀), the potential ofthe third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potentialV₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3200.

When including a transistor using an oxide semiconductor and having alow off-state current, the semiconductor device described above canretain stored data for a long time. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Furthermore, in the semiconductor device, high voltage is not needed forwriting data and deterioration of elements is less likely to occur.Unlike in a conventional nonvolatile memory, for example, it is notnecessary to inject and extract electrons into and from a floating gate;thus, a problem such as deterioration of an insulator is not caused.That is, unlike a conventional nonvolatile memory, the semiconductordevice of one embodiment of the present invention does not have a limiton the number of times data can be rewritten, and the reliabilitythereof is drastically improved. Furthermore, data is written dependingon the state of the transistor (on or off), whereby high-speed operationcan be easily achieved.

<Structure 1 of Semiconductor Device>

The semiconductor device of one embodiment of the present inventionincludes the transistor 3300, the transistor 3200, and the capacitor3100 as shown in FIG. 68. The transistor 3200 is provided over thetransistor 3300, and the capacitor 3100 is provided over the transistor3300 and the transistor 3200.

The transistor 3300 is provided on a substrate 3311 and includes aconductor 3316, an insulator 3314, a semiconductor region 3312 that ispart of the substrate 3311, and low-resistance regions 3318 a and 3318 bfunctioning as a source region and a drain region.

The transistor 3300 may be a p-channel transistor or an n-channeltransistor.

It is preferable that a region of the semiconductor region 3312 where achannel is formed, a region in the vicinity thereof, the low-resistanceregions 3318 a and 3318 b functioning as a source region and a drainregion, and the like contain a semiconductor such as a silicon-basedsemiconductor, more preferably single crystal silicon. Alternatively, amaterial including germanium (Ge), silicon germanium (SiGe), galliumarsenide (GaAs), gallium aluminum arsenide (GaAlAs), or the like may becontained. Silicon whose effective mass is controlled by applying stressto the crystal lattice and thereby changing the lattice spacing may becontained. Alternatively, the transistor 3300 may be ahigh-electron-mobility transistor (HEMT) with GaAs and GaAlAs or thelike.

The low-resistance regions 3318 a and 3318 b contain an element whichimparts n-type conductivity, such as arsenic or phosphorus, or anelement which imparts p-type conductivity, such as boron, in addition toa semiconductor material used for the semiconductor region 3312.

The conductor 3316 functioning as a gate electrode can be formed using asemiconductor material such as silicon containing the element whichimparts n-type conductivity, such as arsenic or phosphorus, or theelement which imparts p-type conductivity, such as boron, or aconductive material such as a metal material, an alloy material, or ametal oxide material.

Note that a work function is determined by a material of the conductor,whereby the threshold voltage can be adjusted. Specifically, it ispreferable to use titanium nitride, tantalum nitride, or the like as theconductor. Furthermore, in order to ensure the conductivity andembeddability of the conductor, it is preferable to use a laminatedlayer of metal materials such as tungsten and aluminum as the conductor.In particular, tungsten is preferable in terms of heat resistance.

In the transistor 3300 shown in FIG. 68, the semiconductor region 3312(part of the substrate 3311) in which a channel is formed includes aprotruding portion. Furthermore, the conductor 3316 is provided to coverside and top surfaces of the semiconductor region 3312 with theinsulator 3314 positioned therebetween. Note that the conductor 3316 maybe formed using a material for adjusting the work function. Thetransistor 3300 having such a structure is also referred to as aFIN-type transistor because the protruding portion of the semiconductorsubstrate is utilized. An insulator serving as a mask for forming theprotruding portion may be provided in contact with a top surface of theprotruding portion. Although the case where the protruding portion isformed by processing part of the semiconductor substrate is describedhere, a semiconductor film having a protruding shape may be formed byprocessing an SOI substrate.

Note that the transistor 3300 shown in FIG. 68 is just an example and isnot limited to the structure shown therein; an appropriate transistormay be used in accordance with a circuit configuration or a drivingmethod. For example, the transistor 3300 may be a planar transistor tobe described later. In the case of using the circuit configuration shownin FIG. 67B, the transistor 3300 may be omitted.

An insulator 3320, an insulator 3322, an insulator 3324, and aninsulator 3326 are stacked sequentially and cover the transistor 3300.

The insulator 3320, the insulator 3322, the insulator 3324, and theinsulator 3326 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, aluminum nitride, or thelike.

The insulator 3322 functions as a planarization film for eliminating alevel difference caused by the transistor 3300 or the like underlyingthe insulator 3322. A top surface of the insulator 3322 may beplanarized by planarization treatment using a CMP method or the like toincrease the level of planarity.

The insulator 3324 is preferably formed using, for example, a filmhaving a barrier property that prevents hydrogen or impurities fromdiffusing from the substrate 3311, the transistor 3300, or the like intoa region where the transistor 3200 is formed.

As an example of the film having a barrier property against hydrogen,silicon nitride formed by a CVD method can be given. Diffusion ofhydrogen into a semiconductor element including an oxide semiconductor,such as the transistor 3200, degrades the characteristics of thesemiconductor element in some cases. Therefore, a film that preventshydrogen diffusion is preferably provided between the transistor 3200and the transistor 3300. Specifically, the film that prevents hydrogendiffusion is a film from which hydrogen is less likely to be released.

The amount of released hydrogen can be measured by TDS, for example. Theamount of hydrogen released from the insulator 3324 that is convertedinto hydrogen atoms per area of the insulator 3324 is less than or equalto 10×10¹⁵ atoms/cm², preferably less than or equal to 5×10¹⁵ atoms/cm²in TDS in the range of 50° C. to 500° C., for example.

Note that the relative dielectric constant of the insulator 3326 ispreferably lower than that of the insulator 3324. For example, therelative dielectric constant of the insulator 3326 is preferably lowerthan 4, more preferably lower than 3. For example, the relativedielectric constant of the insulator 3324 is preferably 0.7 times orless that of the insulator 3326, more preferably 0.6 times or less thatof the insulator 3326. In the case where a material with a low relativedielectric constant is used as an interlayer film, the parasiticcapacitance between wirings can be reduced.

A conductor 3328, a conductor 3330, and the like that are electricallyconnected to the capacitor 3100 or the transistor 3200 are embedded inthe insulator 3320, the insulator 3322, the insulator 3324, and theinsulator 3326. Note that the conductor 3328 and the conductor 3330 eachfunction as a plug or a wiring. Note that a plurality of structures ofconductors functioning as plugs or wirings are collectively denoted bythe same reference numeral in some cases, as described later.Furthermore, in this specification and the like, a wiring and a plugelectrically connected to the wiring may be a single component. That is,there are cases where a part of a conductor functions as a wiring and apart of a conductor functions as a plug.

As a material of each of plugs and wirings (e.g., the conductor 3328 andthe conductor 3330), a conductive material such as a metal material, analloy material, a metal nitride material, or a metal oxide material canbe used in a single-layer structure or a stacked-layer structure. It ispreferable to use a high-melting-point material that has both heatresistance and conductivity, such as tungsten or molybdenum, and it isparticularly preferable to use tungsten. Alternatively, a low-resistanceconductive material such as aluminum or copper is preferably used. Theuse of a low-resistance conductive material can reduce wiringresistance.

A wiring layer may be provided over the insulator 3326 and the conductor3330. For example, in FIG. 68, an insulator 3350, an insulator 3352, andan insulator 3354 are stacked sequentially. Furthermore, a conductor3356 is formed in the insulator 3350, the insulator 3352, and theinsulator 3354. The conductor 3356 functions as a plug or a wiring. Notethat the conductor 3356 can be formed using a material similar to thatused for forming the conductor 3328 and the conductor 3330.

Note that for example, the insulator 3350 is preferably formed using aninsulator having a barrier property with respect to hydrogen, like theinsulator 3324. Furthermore, the conductor 3356 preferably includes aconductor having a barrier property against hydrogen. The conductorhaving a barrier property against hydrogen is formed particularly in anopening of the insulator 3350 having a barrier property againsthydrogen. In such a structure, the transistor 3300 and the transistor3200 can be separated by a barrier layer, so that diffusion of hydrogenfrom the transistor 3300 to the transistor 3200 can be prevented.

Note that as the conductor having a barrier property against hydrogen,tantalum nitride may be used, for example. By stacking tantalum nitrideand tungsten, which has high conductivity, diffusion of hydrogen fromthe transistor 3300 can be prevented while the conductivity of a wiringis ensured. In this case, a tantalum nitride layer having a barrierproperty against hydrogen is preferably in contact with the insulator3350 having a barrier property against hydrogen.

An insulator 3358, an insulator 3210, an insulator 3212, and aninsulator 3216 are stacked sequentially over the insulator 3354. Amaterial having a barrier property against oxygen and hydrogen ispreferably used for one or all of the insulator 3358, the insulator3210, the insulator 3212, and the insulator 3216.

The insulator 3358 and the insulator 3212 are preferably formed using,for example, a film having a barrier property that prevents hydrogen andimpurities from diffusing from a region where the substrate 3311 or thetransistor 3300 is formed or the like into a region where the transistor3200 is formed. Therefore, the insulators 3358 and 3212 can be formedusing a material similar to that used for forming the insulator 3324.

As an example of the film having a barrier property against hydrogen,silicon nitride formed by a CVD method can be given. Diffusion ofhydrogen into a semiconductor element including an oxide semiconductor,such as the transistor 3200, degrades the characteristics of thesemiconductor element in some cases. Therefore, a film that preventshydrogen diffusion is preferably provided between the transistor 3200and the transistor 3300. Specifically, the film that prevents hydrogendiffusion is a film from which hydrogen is less likely to be released.

For example, the insulators 3210 and 3216 can be formed using a materialsimilar to that used for forming the insulator 3320. For example, asilicon oxide film, a silicon oxynitride film, or the like can be usedas the insulator 3216.

A conductor 3218, a conductor forming the transistor 3200, and the likeare embedded in the insulators 3358, 3210, 3212, and 3216. Note that theconductor 3218 functions as a plug or a wiring that is electricallyconnected to the capacitor 3100 or the transistor 3300. The conductor3218 can be formed using a material similar to that used for forming theconductor 3328 and the conductor 3330.

In particular, part of the conductor 3218 which is in contact with theinsulators 3358 and 3212 is preferably a conductor with a barrierproperty against oxygen, hydrogen, and water. When a conductor 3205 witha barrier property against oxygen, hydrogen, and water is provided tocover the conductor 3218, the transistors 3300 and 3200 can becompletely separated by the layer with a barrier property againstoxygen, hydrogen, and water. As a result, diffusion of hydrogen from thetransistor 3300 to the transistor 3200 can be prevented.

An insulator 3224 is provided over the conductor 3205 and the insulator3216. The insulator 3224 functions as a gate insulator of the transistor3200. Although the insulator 3224 contains excess oxygen in some cases,the excess oxygen is blocked by the conductor 3205 with a barrierproperty against oxygen, hydrogen, and water; therefore, the excessoxygen can be prevented from diffusing to the conductor 3218, so thatoxidation of the conductor 3218 can be prevented.

The transistor 3200 is provided over the insulator 3216. Note that, forexample, the structure of the transistor described in the aboveembodiment can be used as the structure of the transistor 3200. Notethat the transistor 3200 in FIG. 68 are just an example and is notlimited to the structure shown therein; an appropriate transistor may beused in accordance with a circuit structure or a driving method.

An insulator 3272 and an insulator 3280 are provided over the transistor3200. The insulator 3280 preferably includes oxide containing oxygen inexcess of that in the stoichiometric composition. That is, in theinsulator 3280, a region containing oxygen in excess of that in thestoichiometric composition (hereinafter also referred to as anoxygen-excess region) is preferably formed. In particular, in the casewhere an oxide semiconductor is used in the transistor 3200, when aninsulator including an oxygen-excess region is provided in an interlayerfilm or the like in the vicinity of the transistor 3200, oxygenvacancies in the transistor 3200 are reduced, whereby the reliabilitycan be improved.

As the insulator including the oxygen-excess region, specifically, anoxide material that releases part of oxygen by heating is preferablyused. Oxide that releases part of oxygen by heating is an oxide film ofwhich the amount of released oxygen converted into oxygen atoms isgreater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greater than orequal to 3.0×10²⁰ atoms/cm³ in TDS. Note that the temperature of thefilm surface in the TDS is preferably higher than or equal to 100° C.and lower than or equal to 700° C., or higher than or equal to 100° C.and lower than or equal to 500° C.

For example, as such a material, a material containing silicon oxide orsilicon oxynitride is preferably used. Alternatively, a metal oxide canbe used. Note that in this specification, “silicon oxynitride” refers toa material that contains oxygen at a higher proportion than nitrogen,and “silicon nitride oxide” refers to a material that contains nitrogenat a higher proportion than oxygen.

The insulator 3280 covering the transistor 3200 may function as aplanarization film that covers a roughness thereunder. An insulator 3282and an insulator 3284 are stacked sequentially over the insulator 3280.

A material having a barrier property against oxygen or hydrogen ispreferably used for one or both of the insulator 3282 and the insulator3284. Thus, the insulator 3282 can be formed using a material similar tothat used for forming the insulator 3212. The insulator 3284 can beformed using an insulator similar to that used for forming the insulator3212. The insulator 3284 can be formed using an insulator similar tothat used for forming the insulator 3216.

For example, when the conductor 3285 is formed to have a layeredstructure, the conductor 3285 preferably includes a conductor with highoxidation resistance. In particular, a conductor with high oxidationresistance is preferably provided in a region in contact with theinsulator 3280 including the oxygen-excess region. Such a structure canprevent the conductor 3285 from absorbing excess oxygen from theinsulator 3280. Furthermore, the conductor 3285 preferably includes aconductor having a barrier property against hydrogen. In particular, aconductor having a barrier property against an impurity such as hydrogenis provided in a region in contact with the insulator 3280 including theoxygen-excess region, whereby diffusion of the impurity of the conductor3285, diffusion of part of the conductor 3285, and diffusion of animpurity from the outside through the conductor 3285 can be prevented.

An conductor 3116 is provided over a conductor 3112 with insulators3130, 3132, and 3134 positioned therebetween. Note that the conductor3116 can be formed using a conductive material such as a metal material,an alloy material, or a metal oxide material. It is preferable to use ahigh-melting-point material which has both heat resistance andconductivity, such as tungsten or molybdenum, and it is particularlypreferable to use tungsten. In the case where the conductor 3116 isformed concurrently with another component such as a conductor, Cu(copper), Al (aluminum), or the like which is a low-resistance metalmaterial may be used.

As illustrated in FIG. 68, the conductor 3116 is provided to cover thetop and side surfaces of the conductor 3112 with the insulators 3130,3132, and 3134 positioned therebetween. That is, a capacitance is formedalso on the side surface of the conductor 3112, so that a capacitanceper projected area of a capacitor can be increased. Thus, thesemiconductor device can be reduced in area, highly integrated, andminiaturized.

An insulator 3150 is provided over the conductor 3116 and the insulator3134. The insulator 3150 can be formed using a material similar to thatused for forming the insulator 3320. The insulator 3150 covering thecapacitor 3100 may function as a planarization film that covers aroughness thereunder.

Note that in this structure, when the conductor 3112 is formed, it ispreferable to remove the top surface of the insulator 3284 so that thedepth of the removed part is greater than the total thickness of theinsulators 3130, 3132, and 3134. For example, by performing over-etchingtreatment, part of the insulator 3284 can be removed concurrently.Furthermore, by forming the conductor 3112 or the like by over-etchingtreatment, etching can be performed without leaving an etching residue.

By changing the kind of etching gas in the etching treatment, part ofthe insulator 3284 can be removed efficiently.

After the conductor 3112 and the conductor 3287 are formed, part of theinsulator 3284 may be removed using the conductor 3112 and the conductor3287 as a hard mask, for example.

After the conductor 3112 is formed, a surface of the conductor 3112 maybe subjected to cleaning treatment. By the cleaning treatment, anetching residue or the like can be removed.

In this structure, the transistor 3200 and the insulator 3216 includingthe oxygen-excess region can be positioned between the insulator 3212and the insulator 3272. The insulators 3212 and 3272 have a barrierproperty that prevents diffusion of oxygen or impurities such ashydrogen and water.

Thus, oxygen released from the insulator 3216 and the transistor 3200can be prevented from diffusing into the capacitor 3100 or the layerwhere the transistor 3300 is formed. Furthermore, impurities such ashydrogen and water can be prevented from diffusing from the layer overthe insulator 3272 and the layer under the insulator 3212 into thetransistor 3200.

That is, oxygen can be efficiently supplied from the oxygen-excessregion of the insulator 3216 to the oxide where the channel is formed inthe transistor 3200, so that oxygen vacancies can be reduced. Moreover,oxygen vacancies can be prevented from being formed by impurities in theoxide where the channel is formed in the transistor 3200. Thus, theoxide where a channel is formed in the transistor 3200 can be an oxidesemiconductor with a low density of defect states and stablecharacteristics. That is, a change in electrical characteristics of thetransistor 3200 can be prevented and the reliability can be improved.

In such a structure, the transistor 3200 and the insulator 3280 can beenclosed tightly. Thus, the oxide where the channel is formed in thetransistor 3200 can be an oxide semiconductor with a low density ofdefect states and stable characteristics. That is, a change inelectrical characteristics of the transistor 3200 can be prevented andthe reliability can be improved.

Modification Example

FIG. 69 illustrates a modification example of this embodiment. FIG. 69is different from FIG. 68 in the structure of the transistor 3300.

In the transistor 3300 illustrated in FIG. 69, the semiconductor region3312 (part of the substrate 3311) in which the channel is formed has aprotruding portion. Furthermore, the conductor 3316 is provided to coverthe top and side surfaces of the semiconductor region 3312 with theinsulator 3314 positioned therebetween. Note that the conductor 3316 maybe formed using a material for adjusting the work function. Thetransistor 3300 having such a structure is also referred to as a FINtransistor because the protruding portion of the semiconductor substrateis utilized. An insulator serving as a mask for forming the protrudingportion may be provided in contact with a top surface of the protrudingportion. Although the case where the protruding portion is formed byprocessing part of the semiconductor substrate is described here, asemiconductor film having a protruding shape may be formed by processingan SOI substrate.

The above is the description of the structure example. With the use ofthe structure, a change in electrical characteristics can be preventedand reliability can be improved in a semiconductor device including atransistor including an oxide semiconductor. A transistor including anoxide semiconductor with high on-state current can be provided. Atransistor including an oxide semiconductor with low off-state currentcan be provided. A semiconductor device with low power consumption canbe provided.

Embodiment 7

In this embodiment, an example of a circuit of a semiconductor deviceincluding the transistor of one embodiment of the present invention orthe like will be described.

<Circuit>

Examples of a circuit of a semiconductor device including the transistoror the like of one embodiment of the present invention will be describedwith reference to FIG. 70 and FIG. 71.

<Memory Device 1>

The semiconductor device in FIG. 70 is different from the semiconductordevice described in the above embodiment in that a transistor 3400 and asixth wiring 3006 are included. Also in this case, data can be writtenand retained in a manner similar to that of the semiconductor devicedescribed in the above embodiment. A transistor similar to thetransistor 3300 described above can be used as the transistor 3400.

The sixth wiring 3006 is electrically connected to a gate of thetransistor 3400, one of a source and a drain of the transistor 3400 iselectrically connected to a drain of the transistor 3300, and the otherof the source and the drain of the transistor 3400 is electricallyconnected to the third wiring 3003.

<Memory Device 2>

A modification example of the semiconductor device (memory device) isdescribed with reference to a circuit diagram in FIG. 71.

The semiconductor device illustrated in FIG. 71 includes transistors4100, 4200, 4300, and 4400 and capacitors 4500 and 4600. Here, atransistor similar to the above-described transistor 3300 can be used asthe transistor 4100, and transistors similar to the above-describedtransistor 3200 can be used as the transistors 4200 to 4400. Capacitorssimilar to the above-described capacitor 3100 can be used as thecapacitors 4500 and 4600. Although not illustrated in FIG. 71, aplurality of the semiconductor devices in FIG. 71 are provided in amatrix. The semiconductor device in FIG. 71 can control writing andreading of a data voltage in accordance with a signal or a potentialsupplied to a wiring 4001, a wiring 4003, and wirings 4005 to 4009.

One of a source and a drain of the transistor 4100 is connected to thewiring 4003. The other of the source and the drain of the transistor4100 is connected to the wiring 4001. Although the transistor 4100 is ap-channel transistor in FIG. 71, the transistor 4100 may be an n-channeltransistor.

The semiconductor device in FIG. 71 includes two data retentionportions. For example, a first data retention portion retains a chargebetween one of a source and a drain of the transistor 4400, oneelectrode of the capacitor 4600, and one of a source and a drain of thetransistor 4200 which are connected to a node FG1. A second dataretention portion retains a charge between a gate of the transistor4100, the other of the source and the drain of the transistor 4200, oneof a source and a drain of the transistor 4300, and one electrode of thecapacitor 4500 which are connected to a node FG2.

The other of the source and the drain of the transistor 4300 isconnected to the wiring 4003. The other of the source and the drain ofthe transistor 4400 is connected to the wiring 4001. A gate of thetransistor 4400 is connected to the wiring 4005. A gate of thetransistor 4200 is connected to the wiring 4006. A gate of thetransistor 4300 is connected to the wiring 4007. The other electrode ofthe capacitor 4600 is connected to the wiring 4008. The other electrodeof the capacitor 4500 is connected to the wiring 4009.

The transistors 4200, 4300, and 4400 each function as a switch forcontrol of writing a data voltage and retaining a charge. Note that, aseach of the transistors 4200, 4300, and 4400, it is preferable to use atransistor having a low current that flows between a source and a drainin an off state (low off-state current). As an example of the transistorwith a low off-state current, a transistor including an oxidesemiconductor in its channel formation region (an OS transistor) ispreferably used. Some advantages of an OS transistor are that it has alow off-state current and can be manufactured to overlap with atransistor including silicon, for example. Although the transistors4200, 4300, and 4400 are n-channel transistors in FIG. 71, thetransistors 4200, 4300, and 4400 may be p-channel transistors.

The transistor 4200 and the transistor 4300 are preferably provided in alayer different from the layer where the transistor 4400 is providedeven when the transistor 4200, the transistor 4300, and the transistor4400 are transistors including oxide semiconductors. In other words, inthe semiconductor device in FIG. 71, the transistor 4100, the transistor4200 and the transistor 4300, and the transistor 4400 are preferablystacked. Layers including transistors may be stacked. That is, byintegrating the transistors, the circuit area can be reduced, so thatthe size of the semiconductor device can be reduced.

Next, operation of writing data to the semiconductor device illustratedin FIG. 71 is described.

First, operation of writing a data voltage to the data retention portionconnected to the node FG1 (hereinafter referred to as writing operation1) is described. In the following description, the data voltage writtento the data retention portion connected to the node FG1 is referred toas V_(D1), and the threshold voltage of the transistor 4100 is referredto as V_(th).

In the writing operation 1, the wiring 4003 is set at V_(D1), and afterthe wiring 4001 is set at a ground potential, the wiring 4001 is broughtinto an electrically floating state. The wirings 4005 and 4006 are setat a high level. The wirings 4007 to 4009 are set at a low level. Then,the potential of the node FG2 in the electrically floating state isincreased, so that a current flows through the transistor 4100. By thecurrent flow, the potential of the wiring 4001 is increased. Thetransistors 4400 and 4200 are turned on. Thus, as the potential of thewiring 4001 is increased, the potentials of the nodes FG1 and FG2 areincreased. When the potential of the node FG2 is increased and a voltage(V_(gs)) between the gate and the source of the transistor 4100 reachesthe threshold voltage V_(th) of the transistor 4100, the current flowingthrough the transistor 4100 is decreased. Accordingly, the increase inthe potentials of the wiring 4001 and the nodes FG1 and FG2 is stopped,so that the potentials of the nodes FG1 and FG2 are fixed at“V_(D1)−V_(th),” which is lower than V_(D1) by V_(th).

In other words, when a current flows through the transistor 4100, V_(D1)supplied to the wiring 4003 is supplied to the wiring 4001, so that thepotentials of the nodes FG1 and FG2 are increased. When the potential ofthe node FG2 becomes “V_(D1)−V_(th)” with the increase in thepotentials, V_(gs) of the transistor 4100 becomes V_(th), so that thecurrent flow is stopped.

Next, operation of writing a data voltage to the data retention portionconnected to the node FG2 (hereinafter referred to as writing operation2) is described. In the following description, the data voltage writtento the data retention portion connected to the node FG2 is referred toas V_(D2).

In the writing operation 2, the wiring 4001 is set at V_(D2), and afterthe wiring 4003 is set at a ground potential, the wiring 4003 is broughtinto an electrically floating state. The wiring 4007 is set at the highlevel. The wirings 4005, 4006, 4008, and 4009 are set at the low level.The transistor 4300 is turned on, so that the wiring 4003 is set at thelow level. Thus, the potential of the node FG2 is also decreased to thelow level, so that the current flows through the transistor 4100. By thecurrent flow, the potential of the wiring 4003 is increased. Thetransistor 4300 is turned on. Thus, as the potential of the wiring 4003is increased, the potential of the node FG2 is increased. When thepotential of the node FG2 is increased and V_(gs) of the transistor 4100becomes V_(th) of the transistor 4100, the current flowing through thetransistor 4100 is decreased. Accordingly, the increase in thepotentials of the wiring 4003 and the node FG2 is stopped, so that thepotential of the node FG2 is fixed at “V_(D2)−V_(th),” which is lowerthan V_(D2) by V_(th).

In other words, when a current flows through the transistor 4100, V_(D2)supplied to the wiring 4001 is supplied to the wiring 4003, so that thepotential of the node FG2 is increased. When the potential of the nodeFG2 becomes “V_(D2)−V_(th)” with the increase in the potential, V_(gs)of the transistor 4100 becomes V_(th), so that the current flow isstopped. At this time, the transistors 4200 and 4400 are off and thepotential of the node FG1 remains at “V_(D1)−V_(th)” written in thewriting operation 1.

In the semiconductor device in FIG. 71, after data voltages are writtento the plurality of data retention portions, the wiring 4009 is set atthe high level, so that the potentials of the nodes FG1 and FG2 areincreased. Then, the transistors are turned off to stop the movement ofcharge; thus, the written data voltages are retained.

By the above-described writing operations of the data voltages to thenodes FG1 and FG2, the data voltages can be retained in the plurality ofdata retention portions. Although examples where “V_(D1)−V_(th)” and“V_(D2)−V_(th)” are used as the written potentials are described, theyare data voltages corresponding to multi-level data. Therefore, in thecase where the data retention portions each retain 4-bit data, 16-level“V_(D1)−V_(th)” and 16-level “V_(D2)−V_(th)” can be obtained.

Next, operation of reading data from the semiconductor deviceillustrated in FIG. 71 is described.

First, operation of reading a data voltage from the data retentionportion connected to the node FG2 (hereinafter referred to as readingoperation 1) is described.

In the reading operation 1, the wiring 4003 which is brought into anelectrically floating state after precharge is discharged. The wirings4005 to 4008 are set at the low level. When the wiring 4009 is set atthe low level, the potential of the node FG2 which is electricallyfloating is set at “V_(D2)−V_(th)”. The potential of the node FG2 isdecreased, so that a current flows through the transistor 4100. By thecurrent flow, the potential of the wiring 4003 which is electricallyfloating is decreased. As the potential of the wiring 4003 is decreased,V_(gs) of the transistor 4100 is decreased. When V_(gs) of thetransistor 4100 becomes V_(th) of the transistor 4100, the currentflowing through the transistor 4100 is decreased. In other words, thepotential of the wiring 4003 becomes “V_(D2),” which is higher than thepotential “V_(D2)−V_(th)” of the node FG2 by V_(th). The potential ofthe wiring 4003 corresponds to the data voltage of the data retentionportion connected to the node FG2. The read analog data voltage issubjected to A/D conversion, so that data of the data retention portionconnected to the node FG2 is obtained.

In other words, the wiring 4003 after precharge is brought into afloating state and the potential of the wiring 4009 is changed from thehigh level to the low level, whereby a current flows through thetransistor 4100. When the current flows, the potential of the wiring4003 which is in a floating state is decreased to be “V_(D2)”. In thetransistor 4100, V_(gs) between “V_(D2)−V_(th)” of the node FG2 and“V_(D2)” of the wiring 4003 becomes V_(th), so that the current stops.Then, “V_(D2)” written in the writing operation 2 is read to the wiring4003.

After data in the data retention portion connected to the node FG2 isobtained, the transistor 4300 is turned on to discharge “V_(D2)−V_(th)”of the node FG2.

Then, the charges retained in the node FG1 are distributed between thenode FG1 and the node FG2, so that data voltage in the data retentionportion connected to the node FG1 is transferred to the data retentionportion connected to the node FG2. The wirings 4001 and 4003 are set atthe low level. The wiring 4006 is set the high level. The wiring 4005and the wirings 4007 to 4009 are set at the low level. When thetransistor 4200 is turned on, the charges in the node FG1 aredistributed between the node FG1 and the node FG2.

Here, the potential after the charge distribution is decreased from thewritten potential “V_(D1)−V_(th)”. Thus, the capacitance of thecapacitor 4600 is preferably larger than the capacitance of thecapacitor 4500. Alternatively, the potential “V_(D1)−V_(th)” written tothe node FG1 is preferably higher than the potential “V_(D2)−V_(th)”corresponding to the same data. By changing the ratio of thecapacitances and setting the written potential higher in advance asdescribed above, a decrease in potential after the charge distributioncan be suppressed. The change in potential due to the chargedistribution is described later.

Next, operation of reading data voltage from the data retention portionconnected to the node FG1 (hereinafter referred to as reading operation2) is described.

In the reading operation 2, the wiring 4003 which is brought into anelectrically floating state after precharge is discharged. The wirings4005 to 4008 are set at the low level. The wiring 4009 is set at thehigh level at the time of precharge and then set at the low level. Whenthe wiring 4009 is set at the low level, the node FG2 which iselectrically floating is set at “V_(D1)−V_(th)”. The potential of thenode FG2 is decreased, so that a current flows through the transistor4100. By the current flow, the potential of the wiring 4003 which iselectrically floating is decreased. As the potential of the wiring 4003is decreased, V_(gs) of the transistor 4100 is decreased. When V_(gs) ofthe transistor 4100 becomes V_(th) of the transistor 4100, the currentflowing through the transistor 4100 is decreased. In other words, thepotential of the wiring 4003 becomes “V_(D1),” which is higher than thepotential “V_(D1)−V_(th)” of the node FG2 by V_(th). The potential ofthe wiring 4003 corresponds to the data voltage of the data retentionportion connected to the node FG1. The read analog data voltage issubjected to A/D conversion, so that data of the data retention portionconnected to the node FG1 is obtained. The above is the operation ofreading the data voltage from the data retention portion connected tothe node FG1.

In other words, the wiring 4003 after precharge is brought into afloating state and the potential of the wiring 4009 is changed from thehigh level to the low level, whereby a current flows through thetransistor 4100. When the current flows, the potential of the wiring4003 which is in a floating state is decreased to be “V_(D1)”. In thetransistor 4100, V_(gs) between “V_(D1)−V_(th)” of the node FG2 and“V_(D1)” of the wiring 4003 becomes V_(th), so that the current stops.Then, “V_(D1)” written in the writing operation 1 is read to the wiring4003.

In the above-described reading operations of the data voltages from thenodes FG1 and FG2, the data voltages can be read from the plurality ofdata retention portions. For example, 4-bit (16-level) data is retainedin each of the node FG1 and the node FG2, whereby 8-bit (256-level) datacan be retained in total. Although first to third layers 4021 to 4023are provided in the structure illustrated in FIG. 71, the storagecapacity can be increased by adding layers without increasing the areaof the semiconductor device.

Note that the read potential can be read as a voltage higher than thewritten data voltage by V_(th). Therefore, V_(th) of “V_(D1)−V_(th)” orV_(th) of “V_(D2)−V_(th)” written in the writing operation can becanceled out in reading. As a result, the storage capacity per memorycell can be improved and read data can be close to accurate data; thus,the data reliability becomes excellent.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 8

In this embodiment, circuit configuration examples to which the OStransistors described in the above embodiment can be used are describedwith reference to FIGS. 72A to 72C, FIGS. 73A to 73C, FIGS. 74A and 74B,and FIGS. 75A and 75B.

FIG. 72A is a circuit diagram of an inverter. An inverter 5800 outputs asignal whose logic is inverted from the logic of a signal supplied to aninput terminal IN to an output terminal OUT. The inverter 5800 includesa plurality of OS transistors. A signal S_(BG) can switch electricalcharacteristics of the OS transistors.

FIG. 72B illustrates an example of the inverter 5800. The inverter 5800includes an OS transistor 5810 and an OS transistor 5820. The inverter5800 can be formed using only n-channel transistors; thus, the inverter5800 can be formed at lower cost than an inverter formed using acomplementary metal oxide semiconductor (i.e., a CMOS inverter).

Note that the transistor 3200 of the present invention can be used asthe OS transistor 5810.

Note that the inverter 5800 including the OS transistors can be providedover a CMOS circuit including Si transistors. Since the inverter 5800can be provided so as to overlap with the CMOS circuit, no additionalarea is required for the inverter 5800, and thus, an increase in thecircuit area can be suppressed.

Each of the OS transistors 5810 and 5820 includes a first gatefunctioning as a front gate, a second gate functioning as a back gate, afirst terminal functioning as one of a source and a drain, and a secondterminal functioning as the other of the source and the drain.

The first gate of the OS transistor 5810 is connected to its secondterminal. The second gate of the OS transistor 5810 is connected to awiring that supplies the signal S_(BG). The first terminal of the OStransistor 5810 is connected to a wiring that supplies a voltage V_(DD).The second terminal of the OS transistor 5810 is connected to the outputterminal OUT.

The first gate of the OS transistor 5820 is connected to the inputterminal IN. The second gate of the OS transistor 5820 is connected tothe input terminal IN. The first terminal of the OS transistor 5820 isconnected to the output terminal OUT. The second terminal of the OStransistor 5820 is connected to a wiring that supplies a voltage V_(SS).

FIG. 72C is a timing chart illustrating the operation of the inverter5800. The timing chart in FIG. 72C illustrates changes of a signalwaveform of the input terminal IN, a signal waveform of the outputterminal OUT, a signal waveform of the signal S_(BG), and the thresholdvoltage of the OS transistor 5810 (FET 5810).

The signal S_(BG) can be supplied to the second gate of the OStransistor 5810 to control the threshold voltage of the OS transistor5810.

The signal S_(BG) includes a voltage V_(BG_A) for shifting the thresholdvoltage in the negative direction and a voltage V_(BG_B) for shiftingthe threshold voltage in the positive direction. The threshold voltageof the OS transistor 5810 can be shifted in the negative direction to bea threshold voltage V_(TH_A) when the voltage V_(BG_A) is applied to thesecond gate. The threshold voltage of the OS transistor 5810 can beshifted in the positive direction to be a threshold voltage V_(TH_B)when the voltage V_(BG_B) is applied to the second gate.

To visualize the above description, FIG. 73A shows a V_(g)−I_(d) curve,which is one of indicators of the transistor's electricalcharacteristics.

When a high voltage such as the voltage V_(BG_A) is applied to thesecond gate, the electrical characteristics of the OS transistor 5810can be shifted to match a curve shown by a dashed line 5840 in FIG. 73A.When a low voltage such as the voltage V_(BG_B) is applied to the secondgate, the electrical characteristics of the OS transistor 5810 can beshifted to match a curve shown by a solid line 5841 in FIG. 73A. Asshown in FIG. 73A, switching the signal S_(BG) between the voltageV_(BG_A) and the voltage V_(BG_B) enables the threshold voltage of theOS transistor 5810 to be shifted in the negative direction or thepositive direction.

The shift of the threshold voltage in the positive direction to thethreshold voltage V_(TH_B) can make a current less likely to flow in theOS transistor 5810. FIG. 73B visualizes the state. As illustrated inFIG. 73B, a current I_(B) that flows in the OS transistor 5810 can beextremely low. Thus, when a signal supplied to the input terminal IN isat a high level and the OS transistor 5820 is on (ON), the voltage ofthe output terminal OUT can be sharply decreased.

Since a state in which a current is less likely to flow in the OStransistor 5810 as illustrated in FIG. 73B can be obtained, a signalwaveform 5831 of the output terminal in the timing chart in FIG. 72C canbe made steep. Shoot-through current between the wiring that suppliesthe voltage V_(DD) and the wiring that supplies the voltage V_(SS) canbe low, leading to low-power operation.

The shift of the threshold voltage in the negative direction to thethreshold voltage V_(TH_A) can make a current flow easily in the OStransistor 5810. FIG. 73C visualizes the state. As illustrated in FIG.73C, a current I_(A) flowing at this time can be higher than at leastthe current I_(B). Thus, when a signal supplied to the input terminal INis at a low level and the OS transistor 5820 is off (OFF), the voltageof the output terminal OUT can be increased sharply.

Since a state in which a current is likely to flow in the OS transistor5810 as illustrated in FIG. 73C can be obtained, a signal waveform 5832of the output terminal in the timing chart in FIG. 72C can be madesteep.

Note that the threshold voltage of the OS transistor 5810 is preferablycontrolled by the signal S_(BG) before the state of the OS transistor5820 is switched, i.e., before time T1 or time T2. For example, as inFIG. 72C, it is preferable that the threshold voltage of the OStransistor 5810 be switched from the threshold voltage V_(TH_A) to thethreshold voltage V_(TH_B) before time T1 at which the level of thesignal supplied to the input terminal IN is switched to the high level.Moreover, as in FIG. 72C, it is preferable that the threshold voltage ofthe OS transistor 5810 be switched from the threshold voltage V_(TH_B)to the threshold voltage V_(TH_A) before time T2 at which the level ofthe signal supplied to the input terminal IN is switched to the lowlevel.

Although the timing chart in FIG. 72C illustrates the configuration inwhich the level of the signal S_(BG) is switched in accordance with thesignal supplied to the input terminal IN, a different configuration maybe employed in which voltage for controlling the threshold voltage isheld by the second gate of the OS transistor 5810 in a floating state,for example. FIG. 74A illustrates an example of such a circuitconfiguration.

The circuit configuration in FIG. 74A is the same as that in FIG. 72B,except that an OS transistor 5850 is added. A first terminal of the OStransistor 5850 is connected to the second gate of the OS transistor5810. A second terminal of the OS transistor 5850 is connected to awiring that supplies the voltage V_(BG_B) (or the voltage V_(BG_A)). Afirst gate of the OS transistor 5850 is connected to a wiring thatsupplies a signal SF. A second gate of the OS transistor 5850 isconnected to the wiring that supplies the voltage V_(BG_B) (or thevoltage V_(BG_A)).

The operation with the circuit configuration in FIG. 74A is describedwith reference to a timing chart in FIG. 74B.

The voltage for controlling the threshold voltage of the OS transistor5810 is supplied to the second gate of the OS transistor 5810 beforetime T3 at which the level of the signal supplied to the input terminalIN is switched to a high level. The signal SF is set to a high level andthe OS transistor 5850 is turned on, so that the voltage V_(BG_B) forcontrolling the threshold voltage is supplied to a node N_(BG).

The OS transistor 5850 is turned off after the voltage of the nodeN_(BG) becomes V_(BG_B). Since the off-state current of the OStransistor 5850 is extremely low, the voltage V_(BG_B) held by the nodeN_(BG) can be retained while the OS transistor 5850 remains off and thenode N_(BG) is in a state that is very close to a floating state.Therefore, the number of times the voltage V_(BG_B) is supplied to thesecond gate of the OS transistor 5850 can be reduced and accordingly,the power consumption for rewriting the voltage V_(BG_B) can be reduced.

Although FIG. 72B and FIG. 74A each illustrate the configuration wherethe voltage is supplied to the second gate of the OS transistor 5810 bycontrol from the outside, a different configuration may be employed inwhich voltage for controlling the threshold voltage is generated on thebasis of the signal supplied to the input terminal IN and supplied tothe second gate of the OS transistor 5810, for example. FIG. 75Aillustrates an example of such a circuit configuration.

The circuit configuration in FIG. 75A is the same as that in FIG. 72B,except that a CMOS inverter 5860 is provided between the input terminalIN and the second gate of the OS transistor 5810. An input terminal ofthe CMOS inverter 5860 is connected to the input terminal IN. An outputterminal of the CMOS inverter 5860 is connected to the second gate ofthe OS transistor 5810.

The operation with the circuit configuration in FIG. 75A is describedwith reference to a timing chart in FIG. 75B. The timing chart in FIG.75B illustrates changes of a signal waveform of the input terminal IN, asignal waveform of the output terminal OUT, an output waveform IN_B ofthe CMOS inverter 5860, and the threshold voltage of the OS transistor5810 (FET 5810).

The output waveform IN_B which corresponds to a signal whose logic isinverted from the logic of the signal supplied to the input terminal INcan be used as a signal that controls the threshold voltage of the OStransistor 5810. Thus, the threshold voltage of the OS transistor 5810can be controlled as described with reference to FIGS. 72A to 72C. Forexample, the signal supplied to the input terminal IN is at a high leveland the OS transistor 5820 is turned on at time T4 in FIG. 75B. At thistime, the output waveform IN_B is at a low level. Accordingly, a currentcan be made less likely to flow in the OS transistor 5810; thus, thevoltage of the output terminal OUT can be sharply decreased.

Moreover, the signal supplied to the input terminal IN is at a low leveland the OS transistor 5820 is turned off at time T5 in FIG. 75B. At thistime, the output waveform IN_B is at a high level. Accordingly, acurrent can easily flow in the OS transistor 5810; thus, the voltage ofthe output terminal OUT can be sharply increased.

As described above, in the configuration of the inverter including theOS transistor in this embodiment, the voltage of the back gate isswitched in accordance with the logic of the signal supplied to theinput terminal IN. In such a configuration, the threshold voltage of theOS transistor can be controlled. The control of the threshold voltage ofthe OS transistor by the signal supplied to the input terminal IN cancause a steep change in the voltage of the output terminal OUT.Moreover, shoot-through current between the wirings that supply powersupply voltages can be reduced. Thus, power consumption can be reduced.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 9

In this embodiment, examples of a semiconductor device which includes aplurality of circuits including OS transistors described in the aboveembodiment are described with reference to FIGS. 76A to 76E, FIGS. 77Aand 77B, FIGS. 78A and 78B, FIGS. 79A to 79C, FIGS. 80A and 80B, FIGS.81A to 81C, and FIGS. 82A and 82B.

FIG. 76A is a block diagram of a semiconductor device 5900. Thesemiconductor device 5900 includes a power supply circuit 5901, acircuit 5902, a voltage generation circuit 5903, a circuit 5904, avoltage generation circuit 5905, and a circuit 5906.

The power supply circuit 5901 is a circuit that generates a voltageV_(ORG) used as a reference. The voltage V_(ORG) is not necessarily onevoltage and can be a plurality of voltages. The voltage V_(ORG) can begenerated on the basis of a voltage V₀ supplied from the outside of thesemiconductor device 5900. The semiconductor device 5900 can generatethe voltage V_(ORG) on the basis of one power supply voltage suppliedfrom the outside. Thus, the semiconductor device 5900 can operatewithout the supply of a plurality of power supply voltages from theoutside.

The circuits 5902, 5904, and 5906 operate with different power supplyvoltages. For example, the power supply voltage of the circuit 5902 is avoltage applied on the basis of the voltage V_(ORG) and the voltageV_(SS) (V_(ORG)>V_(SS)). For example, the power supply voltage of thecircuit 5904 is a voltage applied on the basis of a voltage V_(POG) andthe voltage V_(SS) (V_(POG)>V_(ORG)). For example, the power supplyvoltages of the circuit 5906 are voltages applied on the basis of thevoltage V_(ORG), the voltage V_(SS), and a voltage V_(NEG)(V_(ORG)>V_(SS)>V_(NEG)). When the voltage V_(SS) is equal to a ground(GND) potential, the kinds of voltages generated in the power supplycircuit 5901 can be reduced.

The voltage generation circuit 5903 is a circuit that generates thevoltage V_(POG). The voltage generation circuit 5903 can generate thevoltage V_(POG) on the basis of the voltage V_(ORG) supplied from thepower supply circuit 5901. Thus, the semiconductor device 5900 includingthe circuit 5904 can operate on the basis of one power supply voltagesupplied from the outside.

The voltage generation circuit 5905 is a circuit that generates thevoltage V_(NEG). The voltage generation circuit 5905 can generate thevoltage V_(NEG) on the basis of the voltage V_(ORG) supplied from thepower supply circuit 5901. Thus, the semiconductor device 5900 includingthe circuit 5906 can operate on the basis of one power supply voltagesupplied from the outside.

FIG. 76B illustrates an example of the circuit 5904 that operates withthe voltage V_(POG) and FIG. 76C illustrates an example of a waveform ofa signal for operating the circuit 5904.

FIG. 76B illustrates a transistor 5911. A signal supplied to a gate ofthe transistor 5911 is generated on the basis of, for example, thevoltage V_(POG) and the voltage V_(SS). The signal is generated on thebasis of the voltage V_(POG) at the time when the transistor 5911 isturned on and on the basis of the voltage V_(SS) at the time when thetransistor 5911 is turned off. As shown in FIG. 76C, the voltage V_(POG)is higher than the voltage V_(ORG). Therefore, a conducting statebetween a source (S) and a drain (D) of the transistor 5911 can beobtained more surely. As a result, the frequency of malfunction of thecircuit 5904 can be reduced.

FIG. 76D illustrates an example of the circuit 5906 that operates withthe voltage V_(NEG) and FIG. 76E illustrates an example of a waveform ofa signal for operating the circuit 5906.

FIG. 76D illustrates a transistor 5912 having a back gate. A signalsupplied to a gate of the transistor 5912 is generated on the basis of,for example, the voltage V_(ORG) and the voltage V_(SS). The signal isgenerated on the basis of the voltage V_(ORG) at the time when thetransistor 5911 is turned on and on the basis of the voltage V_(SS) atthe time when the transistor 5911 is turned off. A signal supplied tothe back gate of the transistor 5912 is generated on the basis of thevoltage V_(NEG). As shown in FIG. 76E, the voltage V_(NEG) is lower thanthe voltage V_(SS) (GND). Therefore, the threshold voltage of thetransistor 5912 can be controlled so as to be shifted in the positivedirection. Thus, the transistor 5912 can be surely turned off and acurrent flowing between a source (S) and a drain (D) can be reduced. Asa result, the frequency of malfunction of the circuit 5906 can bereduced and power consumption thereof can be reduced.

The voltage V_(NEG) may be directly supplied to the back gate of thetransistor 5912. Alternatively, a signal supplied to the gate of thetransistor 5912 may be generated on the basis of the voltage V_(ORG) andthe voltage V_(NEG) and the generated signal may be supplied to the backgate of the transistor 5912.

FIGS. 77A and 77B illustrate a modification example of FIGS. 76D and76E.

In a circuit diagram illustrated in FIG. 77A, a transistor 5922 whoseconduction state can be controlled by a control circuit 5921 is providedbetween the voltage generation circuit 5905 and the circuit 5906. Thetransistor 5922 is an n-channel OS transistor. The control signal S_(BG)output from the control circuit 5921 is a signal for controlling theconduction state of the transistor 5922. Transistors 5912A and 5912Bincluded in the circuit 5906 are the same OS transistors as thetransistor 5922.

A timing chart in FIG. 77B shows changes in a potential of the controlsignal S_(BG) and a potential of the node N_(BG). The potential of thenode N_(BG) indicates the states of potentials of back gates of thetransistors 5912A and 5912B. When the control signal S_(BG) is at a highlevel, the transistor 5922 is turned on and the voltage of the nodeN_(BG) becomes the voltage V_(NEG). Then, when the control signal S_(BG)is at a low level, the node N_(BG) is brought into an electricallyfloating state. Since the transistor 5922 is an OS transistor, itsoff-state current is low. Accordingly, even when the node N_(BG) is inan electrically floating state, the voltage V_(NEG) which has beensupplied can be held.

FIG. 78A illustrates an example of a circuit configuration applicable tothe above-described voltage generation circuit 5903. The voltagegeneration circuit 5903 illustrated in FIG. 78A is a five-stage chargepump including diodes D1 to D5, capacitors C1 to C5, and an inverterINV. A clock signal CLK is supplied to the capacitors C1 to C5 directlyor through the inverter INV. When a power supply voltage of the inverterINV is a voltage applied on the basis of the voltage V_(ORG) and thevoltage V_(SS), the voltage V_(POG), which has been increased to apositive voltage having a positively quintupled value of the voltageV_(ORG) by application of the clock signal CLK, can be obtained. Notethat a forward voltage of the diodes D1 to D5 is 0 V. A desired voltageV_(POG) can be obtained when the number of stages of the charge pump ischanged.

FIG. 78B illustrates an example of a circuit configuration applicable tothe above-described voltage generation circuit 5905. The voltagegeneration circuit 5905 illustrated in FIG. 78B is a four-stage chargepump including the diodes D1 to D5, the capacitors C1 to C5, and theinverter INV. The clock signal CLK is supplied to the capacitors C1 toC5 directly or through the inverter INV. When a power supply voltage ofthe inverter INV is a voltage applied on the basis of the voltageV_(ORG) and the voltage V_(SS), the voltage V_(NEG), which has beenreduced from GND (i.e., the voltage V_(SS)) to a negative voltage havinga negatively quadrupled value of the voltage V_(ORG) by application ofthe clock signal CLK, can be obtained. Note that a forward voltage ofthe diodes D1 to D5 is 0 V. A desired voltage V_(NEG) can be obtainedwhen the number of stages of the charge pump is changed.

The circuit configuration of the voltage generation circuit 5903 is notlimited to the configuration of the circuit diagram illustrated in FIG.78A. Modification examples of the voltage generation circuit 5903 areshown in FIGS. 79A to 79C and FIGS. 80A and 80B.

A voltage generation circuit 5903A illustrated in FIG. 79A includestransistors M1 to M10, capacitors C11 to C14, and an inverter INV1. Theclock signal CLK is supplied to gates of the transistors M1 to M10directly or through the inverter INV1. By application of the clocksignal CLK, the voltage V_(POG), which has been increased to a positivevoltage having a positively quadrupled value of the voltage V_(ORG), canbe obtained. A desired voltage V_(POG) can be obtained when the numberof stages is changed. In the voltage generation circuit 5903A in FIG.79A, the off-state current of each of the transistors M1 to M10 can below when the transistors M1 to M10 are OS transistors, and leakage ofcharge held in the capacitors C11 to C14 can be inhibited. Accordingly,efficient voltage increase from the voltage V_(ORG) to the voltageV_(POG) is possible.

A voltage generation circuit 5903B illustrated in FIG. 79B includestransistors M11 to M14, capacitors C15 and C16, and an inverter INV2.The clock signal CLK is supplied to gates of the transistors M11 to M14directly or through the inverter INV2. By application of the clocksignal CLK, the voltage V_(POG), which has been increased to a positivevoltage having a positively doubled value of the voltage V_(ORG), can beobtained. In the voltage generation circuit 5903B in FIG. 79B, theoff-state current of each of the transistors M11 to M14 can be low whenthe transistors M11 to M14 are OS transistors, and leakage of chargeheld in the capacitors C15 and C16 can be inhibited. Accordingly,efficient voltage increase from the voltage V_(ORG) to the voltageV_(POG) is possible.

A voltage generation circuit 5903C in FIG. 79C includes an inductor I11,a transistor M15, a diode D6, and a capacitor C17. The conduction stateof the transistor M15 is controlled by a control signal EN. Owing to thecontrol signal EN, the voltage V_(POG) which is obtained by increasingthe voltage V_(ORG) can be obtained. Since the voltage generationcircuit 5903C in FIG. 79C increases the voltage using the inductor I11,the voltage can be increased efficiently.

A voltage generation circuit 5903D in FIG. 80A has a configuration inwhich the diodes D1 to D5 of the voltage generation circuit 5903 in FIG.78A are replaced with diode-connected transistors M16 to M20. In thevoltage generation circuit 5903D in FIG. 80A, the off-state current ofeach of the transistors M16 to M20 can be low when the transistors M16to M20 are OS transistors, and leakage of charge held in the capacitorsC1 to C5 can be inhibited. Thus, efficient voltage increase from thevoltage V_(ORG) to the voltage V_(POG) is possible.

A voltage generation circuit 5903E in FIG. 80B has a configuration inwhich the transistors M16 to M20 of the voltage generation circuit 5903Din FIG. 80A are replaced with transistor M21 to M25 having back gates.In the voltage generation circuit 5903E in FIG. 80B, the back gates canbe supplied with voltages that are the same as those of the gates, sothat the current flowing through the transistors can be increased. Thus,efficient voltage increase from the voltage V_(ORG) to the voltageV_(POG) is possible.

Note that the modification examples of the voltage generation circuit5903 can also be applied to the voltage generation circuit 5905 in FIG.78B. The configurations of a circuit diagram in this case areillustrated in FIGS. 81A to 81C and FIGS. 82A and 82B. In a voltagegeneration circuit 5905A illustrated in FIG. 81A, the voltage V_(NEG)which has been reduced from the voltage V_(SS) to a negative voltagehaving a negatively tripled value of the voltage V_(ORG) by applicationof the clock signal CLK can be obtained. In a voltage generation circuit5905B illustrated in FIG. 81B, the voltage V_(NEG) which has beenreduced from the voltage V_(SS) to a negative voltage having anegatively doubled value of the voltage V_(ORG) by application of theclock signal CLK can be obtained.

The voltage generation circuits 5905A and 5905B and voltage generationcircuits 5905C to 5905E illustrated in FIGS. 81A to 81C and FIGS. 82Aand 82B have configurations formed by changing the voltages applied tothe wirings or the arrangement of the elements of the voltage generationcircuits 5903A to 5903E illustrated in FIGS. 79A to 79C and FIGS. 80Aand 80B. In the voltage generation circuits 5905A to 5905E illustratedin FIGS. 81A to 81C and FIGS. 82A and 82B, as in the voltage generationcircuits 5903A to 5903E, efficient voltage decrease from the voltageV_(SS) to the voltage V_(NEG) is possible.

As described above, in any of the structures of this embodiment, avoltage required for circuits included in a semiconductor device can beinternally generated. Thus, in the semiconductor device, the kinds ofpower supply voltages supplied from the outside can be reduced.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 10

In this embodiment, examples of CPUs including semiconductor devicessuch as the transistor of one embodiment of the present invention andthe above-described memory device will be described.

<Configuration of CPU>

A semiconductor device 5400 shown in FIG. 83 includes a CPU core 5401, apower management unit 5421, and a peripheral circuit 5422. The powermanagement unit 5421 includes a power controller 5402 and a power switch5403. The peripheral circuit 5422 includes a cache 5404 including cachememory, a bus interface (BUS I/F) 5405, and a debug interface (DebugI/F) 5406. The CPU core 5401 includes a data bus 5423, a control unit5407, a PC (program counter) 5408, a pipeline register 5409, a pipelineregister 5410, an ALU (arithmetic logic unit) 5411, and a register file5412. Data is transmitted between the CPU core 5401 and the peripheralcircuit 5422 such as the cache 5404 via the data bus 5423.

The semiconductor device (cell) can be used for many logic circuitstypified by the power controller 5402 and the control unit 5407,particularly for all logic circuits that can be constituted usingstandard cells. Accordingly, the semiconductor device 5400 can be small.The semiconductor device 5400 can have reduced power consumption. Thesemiconductor device 5400 can have a higher operating speed. Thesemiconductor device 5400 can have a smaller power supply voltagevariation.

When p-channel Si transistors and the transistor described in the aboveembodiment which includes an oxide semiconductor (preferably an oxidecontaining In, Ga, and Zn) in a channel formation region are used in thesemiconductor device (cell) and the semiconductor device (cell) is usedin the semiconductor device 5400, the semiconductor device 5400 can besmall. The semiconductor device 5400 can have reduced power consumption.The semiconductor device 5400 can have a higher operating speed.Particularly when the Si transistors are only p-channel ones, themanufacturing cost can be reduced.

The control unit 5407 has functions of decoding and executinginstructions contained in a program such as inputted applications bycontrolling the overall operations of the PC 5408, the pipelineregisters 5409 and 5410, the ALU 5411, the register file 5412, the cache5404, the bus interface 5405, the debug interface 5406, and the powercontroller 5402.

The ALU 5411 has a function of performing a variety of arithmeticoperations such as four arithmetic operations and logic operations.

The cache 5404 has a function of temporarily storing frequently useddata. The PC 5408 is a register having a function of storing an addressof an instruction to be executed next. Note that although not shown inFIG. 83, the cache 5404 is provided with a cache controller forcontrolling the operation of the cache memory.

The pipeline register 5409 has a function of temporarily storinginstruction data.

The register file 5412 includes a plurality of registers including ageneral purpose register and can store data that is read from the mainmemory, data obtained as a result of arithmetic operations in the ALU5411, or the like.

The pipeline register 5410 has a function of temporarily storing dataused for arithmetic operations of the ALU 5411, data obtained as aresult of arithmetic operations of the ALU 5411, or the like.

The bus interface 5405 has a function of a path for data between thesemiconductor device 5400 and various devices outside the semiconductordevice 5400. The debug interface 5406 has a function of a path of asignal for inputting an instruction to control debugging to thesemiconductor device 5400.

The power switch 5403 has a function of controlling supply of a powersupply voltage to various circuits included in the semiconductor device5400 other than the power controller 5402. The above various circuitsbelong to several different power domains. The power switch 5403controls whether the power supply voltage is supplied to the variouscircuits in the same power domain. In addition, the power controller5402 has a function of controlling the operation of the power switch5403.

The semiconductor device 5400 having the above structure is capable ofperforming power gating. A description will be given of an example ofthe power gating operation sequence.

First, by the CPU core 5401, timing for stopping the supply of the powersupply voltage is set in a register of the power controller 5402. Then,an instruction of starting power gating is sent from the CPU core 5401to the power controller 5402. Then, various registers and the cache 5404included in the semiconductor device 5400 start data saving. Then, thepower switch 5403 stops the supply of a power supply voltage to thevarious circuits other than the power controller 5402 included in thesemiconductor device 5400. Then, an interrupt signal is input to thepower controller 5402, whereby the supply of the power supply voltage tothe various circuits included in the semiconductor device 5400 isstarted. Note that a counter may be provided in the power controller5402 to be used to determine the timing of starting the supply of thepower supply voltage regardless of input of an interrupt signal. Next,the various registers and the cache 5404 start data restoration. Then,execution of an instruction is resumed in the control unit 5407.

Such power gating can be performed in the whole processor or one or aplurality of logic circuits included in the processor. Furthermore,power supply can be stopped even for a short time. Consequently, powerconsumption can be reduced at a fine spatial or temporal granularity.

In performing power gating, data held by the CPU core 5401 or theperipheral circuit 5422 is preferably saved in a short time. In thatcase, the power can be turned on or off in a short time, and an effectof saving power becomes significant.

In order that the data held by the CPU core 5401 or the peripheralcircuit 5422 be saved in a short time, the data is preferably saved in aflip-flop circuit itself (referred to as a flip-flop circuit capable ofbackup operation). Furthermore, the data is preferably saved in an SRAMcell itself (referred to as an SRAM cell capable of backup operation).The flip-flop circuit and SRAM cell which are capable of backupoperation preferably include transistors including an oxidesemiconductor (preferably an oxide containing In, Ga, and Zn) in achannel formation region. Consequently, the transistor has a lowoff-state current; thus, the flip-flop circuit and SRAM cell which arecapable of backup operation can retain data for a long time withoutpower supply. When the transistor has a high switching speed, theflip-flop circuit and SRAM cell which are capable of backup operationcan save and restore data in a short time in some cases.

An example of the flip-flop circuit capable of backup operation isdescribed with reference to FIG. 84.

A semiconductor device 5500 shown in FIG. 84 is an example of theflip-flop circuit capable of backup operation. The semiconductor device5500 includes a first memory circuit 5501, a second memory circuit 5502,a third memory circuit 5503, and a read circuit 5504. As a power supplyvoltage, a potential difference between a potential V1 and a potentialV2 is supplied to the semiconductor device 5500. One of the potential V1and the potential V2 is at a high level, and the other is at a lowlevel. An example of the structure of the semiconductor device 5500 whenthe potential V1 is at a low level and the potential V2 is at a highlevel will be described below.

The first memory circuit 5501 has a function of retaining data when asignal D including the data is input in a period during which the powersupply voltage is supplied to the semiconductor device 5500.Furthermore, the first memory circuit 5501 outputs a signal Q includingthe retained data in the period during which the power supply voltage issupplied to the semiconductor device 5500. On the other hand, the firstmemory circuit 5501 cannot retain data in a period during which thepower supply voltage is not supplied to the semiconductor device 5500.That is, the first memory circuit 5501 can be referred to as a volatilememory circuit.

The second memory circuit 5502 has a function of reading the data heldin the first memory circuit 5501 to store (or save) it. The third memorycircuit 5503 has a function of reading the data held in the secondmemory circuit 5502 to store (or save) it. The read circuit 5504 has afunction of reading the data held in the second memory circuit 5502 orthe third memory circuit 5503 to store (or restore) it in the firstmemory circuit 5501.

In particular, the third memory circuit 5503 has a function of readingthe data held in the second memory circuit 5502 to store (or save) iteven in the period during which the power supply voltage is not suppliedto the semiconductor device 5500.

As shown in FIG. 84, the second memory circuit 5502 includes atransistor 5512 and a capacitor 5519. The third memory circuit 5503includes a transistor 5513, a transistor 5515, and a capacitor 5520. Theread circuit 5504 includes a transistor 5510, a transistor 5518, atransistor 5509, and a transistor 5517.

The transistor 5512 has a function of charging and discharging thecapacitor 5519 in accordance with data held in the first memory circuit5501. The transistor 5512 is desirably capable of charging anddischarging the capacitor 5519 at a high speed in accordance with dataheld in the first memory circuit 5501. Specifically, the transistor 5512desirably contains crystalline silicon (preferably polycrystallinesilicon, more preferably single crystal silicon) in a channel formationregion.

The conduction state or the non-conduction state of the transistor 5513is determined in accordance with the charge held in the capacitor 5519.The transistor 5515 has a function of charging and discharging thecapacitor 5520 in accordance with the potential of a wiring 5544 whenthe transistor 5513 is in a conduction state. It is desirable that theoff-state current of the transistor 5515 be extremely low. Specifically,the transistor 5515 desirably contains an oxide semiconductor(preferably an oxide containing In, Ga, and Zn) in a channel formationregion.

Specific connection relations between the elements will be described.One of a source and a drain of the transistor 5512 is connected to thefirst memory circuit 5501. The other of the source and the drain of thetransistor 5512 is connected to one electrode of the capacitor 5519, agate of the transistor 5513, and a gate of the transistor 5518. Theother electrode of the capacitor 5519 is connected to a wiring 5542. Oneof a source and a drain of the transistor 5513 is connected to thewiring 5544. The other of the source and the drain of the transistor5513 is connected to one of a source and a drain of the transistor 5515.The other of the source and the drain of the transistor 5515 isconnected to one electrode of the capacitor 5520 and a gate of thetransistor 5510. The other electrode of the capacitor 5520 is connectedto a wiring 5543. One of a source and a drain of the transistor 5510 isconnected to the wiring 5541. The other of the source and the drain ofthe transistor 5510 is connected to one of a source and a drain of thetransistor 5518. The other of the source and the drain of the transistor5518 is connected to one of a source and a drain of the transistor 5509.The other of the source and the drain of the transistor 5509 isconnected to one of a source and a drain of the transistor 5517 and thefirst memory circuit 5501. The other of the source and the drain of thetransistor 5517 is connected to a wiring 5540. Although a gate of thetransistor 5509 is connected to a gate of the transistor 5517 in FIG.84, it is not necessarily connected to the gate of the transistor 5517.

The transistor described in the above embodiment as an example can beapplied to the transistor 5515. Because of the low off-state current ofthe transistor 5515, the semiconductor device 5500 can retain data for along time without power supply. The favorable switching characteristicsof the transistor 5515 allow the semiconductor device 5500 to performhigh-speed backup and recovery.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 11

In this embodiment, a semiconductor wafer, a chip, and an electroniccomponent of one embodiment of the present invention will be described.

<Semiconductor Wafer and Chip>

FIG. 85A is a top view illustrating a substrate 5711 before dicingtreatment. As the substrate 5711, a semiconductor substrate (alsoreferred to as a “semiconductor wafer”) can be used, for example. Aplurality of circuit regions 5712 are provided over the substrate 5711.A semiconductor device, a CPU, an RF tag, an image sensor, or the likeof one embodiment of the present invention can be provided in thecircuit region 5712.

The plurality of circuit regions 5712 are each surrounded by aseparation region 5713. Separation lines (also referred to as “dicinglines”) 5714 are set at a position overlapping with the separationregion 5713. The substrate 5711 can be cut along the separation lines5714 into chips 5715 including the circuit regions 5712. FIG. 85B is anenlarged view of the chip 5715.

A conductive layer or a semiconductor layer may be provided in theseparation region 5713. Providing a conductive layer or a semiconductorlayer in the separation region 5713 relieves ESD that might be caused ina dicing step, preventing a decrease in the yield of the dicing step. Adicing step is generally performed while letting pure water whosespecific resistance is decreased by dissolution of a carbonic acid gasor the like flow to a cut portion, in order to cool down a substrate,remove swarf, and prevent electrification, for example. Providing aconductive layer or a semiconductor layer in the separation region 5713allows a reduction in the usage of the pure water. Therefore, the costof manufacturing semiconductor devices can be reduced. Thus,semiconductor devices can be manufactured with improved productivity.

For a semiconductor layer provided in the separation region 5713, amaterial having a band gap greater than or equal to 2.5 eV and less thanor equal to 4.2 eV, preferably greater than or equal to 2.7 eV and lessthan or equal to 3.5 eV is preferably used. The use of such a materialallows accumulated charges to be released slowly; thus, the rapid moveof charges due to ESD can be suppressed and electrostatic breakdown isless likely to occur.

<Electronic Component>

FIGS. 86A and 86B show an example where the chip 5715 is used to make anelectronic component. Note that the electronic component is alsoreferred to as a semiconductor package or an IC package. This electroniccomponent has a plurality of standards and names depending on a terminalextraction direction and a terminal shape.

The electronic component is completed when the semiconductor devicedescribed in the above embodiment is combined with components other thanthe semiconductor device in an assembly process (post-process).

The post-process will be described with reference to a flow chart inFIG. 86A. After an element substrate including the semiconductor devicedescribed in the above embodiment is completed in a pre-process, a backsurface grinding step in which a back surface (a surface where thesemiconductor device and the like are not formed) of the elementsubstrate is ground is performed (Step S5721). When the elementsubstrate is thinned by grinding, warpage or the like of the elementsubstrate is reduced, so that the size of the electronic component canbe reduced.

Next, the element substrate is divided into a plurality of chips (chips5715) in a dicing step (Step S5722). Then, the separated chips areindividually picked up to be bonded to a lead frame in a die bondingstep (Step S5723). To bond a chip and a lead frame in the die bondingstep, a method such as bonding with a resin or a tape is selected asappropriate depending on products. Note that the chip may be bonded toan interposer substrate instead of the lead frame.

Next, a wire bonding step for electrically connecting a lead of the leadframe and an electrode on the chip through a metal wire is performed(Step S5724). A silver line or a gold line can be used as the metal fineline. Ball bonding or wedge bonding can be used as the wire bonding.

The wire-bonded chip is subjected to a sealing step (a molding step) ofsealing the chip with an epoxy resin or the like (Step S5725). Throughthe sealing step, the inside of the electronic component is filled witha resin, so that a circuit portion incorporated in the chip and a wirefor connecting the chip to the lead can be protected from externalmechanical force, and deterioration of characteristics (a decrease inreliability) due to moisture or dust can be reduced.

Subsequently, the lead of the lead frame is plated in a lead platingstep (Step S5726). This plating process prevents rust of the lead andfacilitates soldering at the time of mounting the chip on a printedcircuit board in a later step. Then, the lead is cut and processed in ashaping step (Step S5727).

Next, a printing (marking) step is performed on a surface of the package(Step S5728). After a testing step (Step S5729) for checking whether anexternal shape is good and whether there is a malfunction, for example,the electronic component is completed.

FIG. 86B is a schematic perspective diagram of a completed electroniccomponent. FIG. 86B is a schematic perspective diagram illustrating aquad flat package (QFP) as an example of the electronic component. Anelectronic component 5750 in FIG. 86B includes a lead 5755 and asemiconductor device 5753. As the semiconductor device 5753, thesemiconductor device described in the above embodiment or the like canbe used.

The electronic component 5750 in FIG. 86B is mounted on a printedcircuit board 5752, for example. A plurality of electronic components5750 are combined and electrically connected to each other over theprinted circuit board 5752; thus, a substrate on which the electroniccomponents are mounted (a circuit board 5754) is completed. Thecompleted circuit board 5754 is provided in an electronic device or thelike.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 12

In this embodiment, applications of the semiconductor device describedin any of the above embodiments will be described. The semiconductordevice described in any of the above embodiments is preferably used inan electronic device that can withstand high temperatures. Thesemiconductor device described in any of the above embodiments can beused, for example, for a protective circuit of a battery in anelectronic device such as a computer display that can display images;and a protective circuit of a battery provided for an electromagneticcooker or a vehicle (e.g., a bicycle) that is driven with power from afixed power source.

Application examples of a semiconductor device that functions as aprotective circuit will be described with reference to FIGS. 87A to 87C.

FIG. 87A illustrates an electromagnetic cooker 1900 as an applicationexample of a semiconductor device that functions as a protectivecircuit. The electromagnetic cooker 1900 heats cookware and the like byusing electromagnetic induction generated by current flowing through acoil unit 1901. The electromagnetic cooker 1900 includes a battery 1902for supplying current that is to flow through the coil unit 1901, asemiconductor device 1903 functioning as a protective circuit, and asolar battery 1904 for charging the battery 1902. Note that FIG. 87Aillustrates the solar battery 1904 as a means to charge the battery1902; alternatively, the battery 1902 may be charged by another means.The semiconductor device 1903 functioning as a protective circuit canreduce application of overvoltage to the battery 1902 even at hightemperatures. Moreover, the off-state current that flows when theprotective circuit is not operated is extremely low, and thus, the powerconsumption can be reduced.

FIG. 87B illustrates an electric bicycle 1910 as an application exampleof a semiconductor device functioning as a protective circuit. Theelectric bicycle 1910 obtains power when current flows through a motorunit 1911. The electric bicycle 1910 includes a battery 1912 forsupplying current that is to flow through the motor unit 1911 and asemiconductor device 1913 serving as a protective circuit. Although ameans to charge the battery 1912 is not particularly illustrated in FIG.87B, the battery 1912 may be charged by an electric generator or thelike that is additionally provided. The semiconductor device 1913functioning as a protective circuit can reduce application ofovervoltage to the battery 1912 even at high temperatures. Moreover, theoff-state current that flows when the protective circuit is not operatedis extremely low, and thus, the power consumption can be reduced. Notethat a pedal is illustrated in FIG. 87B; however; the pedal is notnecessarily provided.

FIG. 87C illustrates an electric car 1920 as an application example of asemiconductor device functioning as a protective circuit. The electriccar 1920 obtains power when current flows through a motor unit 1921.Moreover, the electric car 1920 includes a battery 1922 for supplyingcurrent that is to flow through the motor unit 1921 and a semiconductordevice 1923 functioning as a protective circuit. Although a means tocharge the battery 1922 is not particularly illustrated in FIG. 87C, thebattery 1922 may be charged by an electric generator or the like that isadditionally provided. The semiconductor device 1923 functioning as aprotective circuit can reduce application of overvoltage to the battery1922 even at high temperatures. Moreover, the off-state current thatflows when the protective circuit is not operated is extremely low, andthus, the power consumption can be reduced.

Note that in this embodiment, what is illustrated in the drawing can befreely combined with or replaced with what is described in anotherembodiment as appropriate.

Example 1

In this example, results of observation and elemental analysis ofIn—Ga—Zn oxide films (hereinafter referred to as IGZO films) formed bythe method described in the above embodiment will be described.

An IGZO film of a sample of this example was formed over a glasssubstrate with the intended thickness set to 100 nm by a sputteringmethod using an In—Ga—Zn oxide target (with an atomic ratio ofIn:Ga:Zn=4:2:4.1). The IGZO film was formed in an atmosphere includingan argon gas at 180 sccm and an oxygen gas at 20 sccm, where thepressure was controlled to 0.6 Pa, the substrate temperature was roomtemperature, and an alternating-current power of 2.5 kW was applied.

The formed IGZO film of the sample was observed by HAADF-STEM and wassubjected to measurement using EDX. With the use of an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.,the HAADF-STEM image was taken and the EDX measurement was performedunder conditions where the acceleration voltage was 200 kV, andirradiation with an electron beam with a diameter of approximately 0.1nmϕ was performed.

In the EDX measurement, an energy dispersive X-ray spectrometerJED-2300T was used as an elemental analysis apparatus. A Si driftdetector was used to detect X-rays emitted from the sample.

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in ananalysis target region of a sample, and the energy of characteristicX-rays of the sample generated by the irradiation and its frequency aremeasured. In this example, peaks of an EDX spectrum of the point wereattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, and electron transition to the Kshell in a Zn atom and the K shell in an O atom, and the proportions ofthe atoms in the point were calculated. An EDX mapping image indicatingdistributions of the proportions of the atoms can be obtained throughthis process in an analysis target region of the sample.

FIGS. 88A to 88E and FIGS. 89A to 89E show the HAADF-STEM images and EDXmapping images of the IGZO film of the sample. FIGS. 88A to 88E show theHAADF-STEM images and EDX mapping images of a plane of the IGZO film,and FIGS. 89A to 89E show those of a cross section of the IGZO film.FIG. 88A and FIG. 89A are the HAADF-STEM images of the sample. FIG. 88Band FIG. 89B are EDX mapping images of O atoms, FIG. 88C and FIG. 89Care EDX mapping images of Zn atoms, FIG. 88D and FIG. 89D are EDXmapping images of Ga atoms, and FIG. 88E and FIG. 89E are EDX mappingimages of In atoms. Note that the HAADF-STEM images and EDX mappingimages in FIGS. 88A to 88E and FIGS. 89A to 89E were taken at amagnification of 7,200,000 times.

Bars above the EDX mapping images shown in FIGS. 88B to 88E and FIGS.89B to 89E indicate the proportions [atomic %] of atoms in some pointsof the IGZO film.

The EDX mapping images in FIGS. 88B to 88E and FIGS. 89B to 89E showrelative distribution of brightness indicating that the atoms havedistributions in the IGZO film. Here, attention is focused on OutlinedSquare 1A and Outlined Square 1B in FIGS. 88B to 88E and Outlined Square2A and Outlined Square 2B in FIGS. 89B to 89E.

In FIG. 88E and FIG. 89E, a relatively bright region occupies a largearea in each of Outlined Squares 1A and 2A, while a relatively darkregion occupies a large area in each of Outlined Squares 1B and 2B. Thatis, the number of In atoms is relatively large in the regions indicatedby Outlined Squares 1A and 2A and the number of In atoms is relativelysmall in the regions indicated by Outlined Squares 1B and 2B. In FIG.88E and FIG. 89E, the relatively bright regions correspond to Regions A1described in the above embodiment, and the relatively dark regionscorrespond to Regions B1 described in the above embodiment.

In contrast to FIG. 88E and FIG. 89E, in FIG. 88D and FIG. 89D, arelatively dark region occupies a large area in each of Outlined Squares1A and 2A, while a relatively bright region occupies a large area ineach of Outlined Squares 1B and 2B. That is, the number of Ga atoms isrelatively small in the regions indicated by Outlined Squares 1A and 2Aand the number of Ga atoms is relatively large in the regions indicatedby Outlined Squares 1B and 2B. In this manner, the number of Ga atomstends to be relatively small in the region including a relatively largenumber of In atoms, and the number of Ga atoms tends to be relativelylarge in the region including a relatively small number of In atoms.Accordingly, in FIG. 88D and FIG. 89D, the relatively bright regionsroughly correspond to Regions B1 described in the above embodiment, andthe relatively dark regions roughly correspond to Regions A1 describedin the above embodiment.

In FIG. 88C and FIG. 89C, a relatively bright region occupies a largearea in each of Outlined Square 1B and Outlined Square 2B, whileOutlined Square 1A and Outlined Square 2A each include a bright regionwhose area is not as large as that of the relatively bright region inOutlined Square 1B and Outlined Square 2B. In other words, the number ofZn atoms is relatively large in the regions indicated by Outlined Square1B and Outlined Square 2B, and the number of Zn atoms in the regionsindicated by Outlined Square 1A and Outlined Square 2A is not as largeas that in the regions indicated by Outlined Square 1B and OutlinedSquare 2B.

As for FIG. 88B and FIG. 89B, similarly, the number of oxygen atoms isrelatively large in the regions indicated by Outlined Square 1A,Outlined Square 1B, Outlined Square 2A, and Outlined Square 2B.

As described above, Region A1 of the IGZO film includes a large numberof In atoms and a large number of O atoms and also includes a smallernumber of Zn atoms than Region B1. It is thus suggested that Region A1has a high content of, for example, indium, indium oxide, and In—Znoxide. Accordingly, Region A1 serves as a region having higherconductivity than Region B1, so that Region A1 contributes to increasesin field-effect mobility and on-state current of a transistor.

Here, when Regions A1 in FIG. 88E and FIG. 89E are examined, a pluralityof particulate portions can be seen in Region A1. Observation of theparticulate portions shows that they have a diameter of greater than orequal to 0.5 nm and less than or equal to 1.5 nm. Regions A1 appear tobe formed by the plurality of particulate portions that are connected toeach other. In this manner, Regions A1 extend in a cloud-like manner.The particulate portions included in Regions A1 correspond to theclusters in Regions A1 described in the above embodiment.

Furthermore, Region B1 of the IGZO film includes a large number of Gaatoms, a large number of Zn atoms, and a large number of O atoms andalso includes a smaller number of In atoms than Region A1. It is thussuggested that Region B1 has a high content of, for example, In—Ga—Znoxide. Accordingly, Region B1 serves as a region having a highersemiconductor property than Region A1, so that Region B1 contributes tothe switching characteristics of a transistor.

Here, when the regions that correspond to Regions B1 in FIG. 88D andFIG. 89D are examined, a plurality of particulate portions can be seenin Region B1. Regions B1 also appear to be formed by the plurality ofparticulate portions that are connected to each other. In this manner,Regions B1 also extend in a cloud-like manner. The particulate portionsincluded in Regions B1 correspond to the clusters in Regions B1described in the above embodiment.

As described above, the IGZO film of the sample fabricated in thisexample is a composite oxide semiconductor where In-rich Regions A1 andIn-poor Regions B1 are formed. Region A1 contributes to the on-statecurrent and field-effect mobility of a transistor, and Region B1contributes to the switching characteristics of a transistor. Therefore,with the use of the composite oxide semiconductor, a transistor withelectrical characteristics in which the on-state current and themobility are high and the S value is small can be manufactured.

At least part of this example can be implemented in combination with anyof the embodiments and the other examples described in thisspecification as appropriate.

Example 2

In this example, a sample (Sample S2) different from Sample S1Adescribed in Embodiment 1 was fabricated, and I_(d)−V_(g)characteristics of Sample S2 were examined.

The conditions used for fabricating Sample S2 in this example aredifferent from those used for fabricating Sample S1A only in theformation conditions of the oxide semiconductor film 108.

The oxide semiconductor film used for Sample S2 was formed under thefollowing conditions: the substrate temperature was room temperature(25° C.); an argon gas with a flow rate of 140 sccm and an oxygen gaswith a flow rate of 60 sccm were introduced into a chamber of thesputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of2.5 kw was applied to a metal oxide target containing indium, gallium,and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1). In this example,the oxygen flow rate percentage in deposition of the oxide semiconductorfilm was 30%.

Next, I_(d)−V_(g) characteristics of the fabricated transistor of SampleS2 were measured. The measurement conditions of the I_(d)−V_(g)characteristics of the transistor were the same as those used formeasuring the I_(d)−V_(g) characteristics of Sample S1A in Embodiment 1.

FIG. 90 shows the I_(d)−V_(g) characteristics of Sample S2. In FIG. 90,the first vertical axis represents I_(d) (A), the second vertical axisrepresents field-effect mobility (μ_(FE)) (cm²/Vs), and the horizontalaxis represents V_(g) (V). Note that the field-effect mobility wasmeasured when V_(d) was 20 V.

The transistor, which is a semiconductor device of one embodiment of thepresent invention, has excellent electrical characteristics as shown inFIG. 90. Here, Table 7 shows the transistor characteristics that areshown in FIG. 90.

TABLE 7 μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/ [cm²V⁻¹s⁻¹] [V][V/decade] [A/cm²] [cm²V⁻¹s⁻¹] μFE(@Vg = 2 V) 70.5 −0.42 0.14 <1 × 10⁻¹²52.2 1.35

As shown in Table 7, Sample S2 includes a first region where the maximumvalue of the field-effect mobility of the transistor at a gate voltageof higher than 0 V and lower than or equal to 10 V is larger than orequal to 40 cm²/Vs and smaller than 150 cm²/Vs, a second region wherethe threshold voltage is higher than or equal to −1 V and lower than orequal to 1 V, a third region where the S value is smaller than 0.3V/decade, and a fourth region where the off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 1 and smaller than 1.5 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents the value of the field-effect mobility of the transistor at agate voltage of 2 V.

The above-described transistor characteristics are obtained by using thecomposite oxide semiconductor or C/IGZO described in Embodiment 1.Therefore, in the case where the composite oxide semiconductor or C/IGZOis used in a semiconductor layer of a transistor, a function ofachieving high field-effect mobility and a function of achievingexcellent switching characteristics can be obtained at the same time.

At least part of this example can be implemented in combination with anyof the embodiments and the other examples described in thisspecification as appropriate.

REFERENCE NUMERALS

A1: region, A2: region, B1: region, B2: region, 82: insulating film, 84:insulating film, 86: insulating film, 88: oxide semiconductor film, 90:structure, 100A: transistor, 100B: transistor, 100C: transistor, 100D:transistor, 100E: transistor, 100F: transistor, 100G: transistor, 100H:transistor, 100J: transistor, 102: substrate, 104: insulating film, 106:conductive film, 108: oxide semiconductor film, 108_1: oxidesemiconductor film, 108_2: oxide semiconductor film, 108_3: oxidesemiconductor film, 108 d: drain region, 108 f: region, 108 i: channelregion, 108 s: source region, 110: insulating film, 112: conductivefilm, 112_1: conductive film, 112_2: conductive film, 116: insulatingfilm, 118: insulating film, 120 a: conductive film, 120 b: conductivefilm, 122: insulating film, 141 a: opening portion, 141 b: openingportion, 143: opening portion, 300A: transistor, 300B: transistor, 300C:transistor, 300D: transistor, 300E: transistor, 300F: transistor, 300G:transistor, 302: substrate, 304: conductive film, 306: insulating film,307: insulating film, 308: oxide semiconductor film, 308_1: oxidesemiconductor film, 308_2: oxide semiconductor film, 308_3: oxidesemiconductor film, 312 a: conductive film, 312 b: conductive film, 312c: conductive film, 314: insulating film, 316: insulating film, 318:insulating film, 320 a: conductive film, 320 b: conductive film, 341 a:opening portion, 341 b: opening portion, 342 a: opening portion, 342 b:opening portion, 342 c: opening portion, 351: opening portion, 352 a:opening portion, 352 b: opening portion, 501: pixel circuit, 502: pixelportion, 504: driver circuit portion, 504 a: gate driver, 504 b: sourcedriver, 506: protection circuit, 507: terminal portion, 550: transistor,552: transistor, 554: transistor, 560: capacitor, 562: capacitor, 570:liquid crystal element, 572: light-emitting element, 700: displaydevice, 701: substrate, 702: pixel portion, 704: source driver circuitportion, 705: substrate, 706: gate driver circuit portion, 708: FPCterminal portion, 710: signal line, 711: wiring portion, 712: sealant,716: FPC, 730: insulating film, 732: sealing film, 734: insulating film,736: coloring film, 738: light-blocking film, 750: transistor, 752:transistor, 760: connection electrode, 770: planarization insulatingfilm, 772: conductive film, 773: insulating film, 774: conductive film,775: liquid crystal element, 776: liquid crystal layer, 778: structure,780: anisotropic conductive film, 782: light-emitting element, 783:droplet discharge apparatus, 784: droplet, 785: layer, 786: EL layer,788: conductive film, 790: capacitor, 791: touch panel, 792: insulatingfilm, 793: electrode, 794: electrode, 795: insulating film, 796:electrode, 797: insulating film, 1400: droplet discharge apparatus,1402: substrate, 1403: droplet discharge means, 1404: imaging means,1405: head, 1406: dotted line, 1407: control means, 1408: storagemedium, 1409: image processing means, 1410: computer, 1411: marker,1412: head, 1413: material supply source, 1414: material supply source,1900: electromagnetic cooker, 1901: coil unit, 1902: battery, 1903:semiconductor device, 1904: solar battery, 1910: electric bicycle, 1911:motor unit, 1912: battery, 1913: semiconductor device, 1920: electriccar, 1921: motor unit, 1922: battery, 1923: semiconductor device, 2190:plasma, 2192: cation, 2194: sputtered particle, 2196: cluster, 2198:cluster, 2500 a: target, 2500 b: target, 2501: deposition chamber, 2502a: region, 2504 a: region, 2510 a: backing plate, 2510 b: backing plate,2520: target holder, 2520 a: target holder, 2520 b: target holder, 2530a: magnet unit, 2530 b: magnet unit, 2530N1: magnet, 2530N2: magnet,2530S: magnet, 2532: magnet holder, 2542: member, 2560: substrate, 2570:substrate holder, 2580 a: magnetic line of force, 2580 b: magnetic lineof force, 3001: wiring, 3002: wiring, 3003: wiring, 3004: wiring, 3005:wiring, 3006: wiring, 3100: capacitor, 3112: conductor, 3116: conductor,3130: insulator, 3132: insulator, 3134: insulator, 3150: insulator,3200: transistor, 3205: conductor, 3210: insulator, 3212: insulator,3216: insulator, 3218: conductor, 3224: insulator, 3272: insulator,3280: insulator, 3282: insulator, 3284: insulator, 3285: conductor,3300: transistor, 3311: substrate, 3312: semiconductor region, 3314:insulator, 3316: conductor, 3318 a: low-resistance region, 3318 b:low-resistance region, 3320: insulator, 3322: insulator, 3324:insulator, 3326: insulator, 3328: conductor, 3330: conductor, 3350:insulator, 3352: insulator, 3354: insulator, 3356: conductor, 3358:insulator, 3400: transistor, 4001: wiring, 4003: wiring, 4005: wiring,4006: wiring, 4007: wiring, 4008: wiring, 4009: wiring, 4021: layer,4023: layer, 4100: transistor, 4200: transistor, 4300: transistor, 4400:transistor, 4500: capacitor, 4600: capacitor, 5400: semiconductordevice, 5401: CPU core, 5402: power control, 5403: power switch, 5404:cache, 5405: bus interface, 5406: debug interface, 5407: control unit,5408: PC, 5409: pipeline register, 5410: pipeline register, 5411: ALU,5412: register file, 5421: power management unit, 5422: peripheralcircuit, 5423: data bus, 5500: semiconductor device, 5501: memorycircuit, 5502: memory circuit, 5503: memory circuit, 5504: circuit,5509: transistor, 5510: transistor, 5512: transistor, 5513: transistor,5515: transistor, 5517: transistor, 5518: transistor, 5519: capacitor,5520: capacitor, 5540: wiring, 5541: wiring, 5542: wiring, 5543: wiring,5544: wiring, 5711: substrate, 5712: circuit region, 5713: separationregion, 5714: separation line, 5715: chip, 5750: electronic component,5752: printed circuit board, 5753: semiconductor device, 5754: circuitboard, 5755: lead, 5800: inverter, 5810: OS transistor, 5820: OStransistor, 5831: signal waveform, 5832: signal waveform, 5840: dashedline, 5841: solid line, 5850: OS transistor, 5860: CMOS inverter, 5900:semiconductor device, 5901: power supply circuit, 5902: circuit, 5903:voltage generation circuit, 5903A: voltage generation circuit, 5903B:voltage generation circuit, 5903C: voltage generation circuit, 5903D:voltage generation circuit, 5903E: voltage generation circuit, 5904:circuit, 5905: voltage generation circuit, 5905A: voltage generationcircuit, 5906: circuit, 5911: transistor, 5912: transistor, 5912A:transistor, 5912B: transistor, 5921: control circuit, 5922: transistor,7000: display module, 7001: upper cover, 7002: lower cover, 7003: FPC,7004: touch panel, 7005: FPC, 7006: display panel, 7007: backlight,7008: light source, 7009: frame, 7010: printed board, 7011: battery,8000: camera, 8001: housing, 8002: display portion, 8003: operationbuttons, 8004: shutter button, 8006: lens, 8100: finder, 8101: housing,8102: display portion, 8103: button, 8200: head-mounted display, 8201:mounting portion, 8202: lens, 8203: main body, 8204: display portion,8205: cable, 8206: battery, 8300: head-mounted display, 8301: housing,8302: display portion, 8304: fixing bands, 8305: lenses, 9000: housing,9001: display portion, 9003: speaker, 9005: operation key, 9006:connection terminal, 9007: sensor, 9008: microphone, 9050: operationbutton, 9051: information, 9052: information, 9053: information, 9054:information, 9055: hinge, 9100: television device, 9101: portableinformation terminal, 9102: portable information terminal, 9150:television device, 9151: housing, 9152: display portion, 9153: stand,9154: remote controller, 9200: portable information terminal, 9201:portable information terminal, 9250: notebook personal computer, 9251:housing, 9252: display portion, 9253: keyboard, 9254: pointing device,9300: slot machine, 9301: housing, 9302: sensor, 9303: display portion,9304: start lever, 9305: stop switch, 9306: light source for sensor,9400: automobile, 9401: car body, 9402: wheel, 9403: windshield, 9404:light, 9405: fog lamp, 9410: display portion, 9411: display portion,9412: display portion, 9413: display portion, 9414: display portion,9415: display portion, 9416: display portion, 9417: display portion,9500: display device, 9501: display panel, 9502: display region, 9503:region, 9511: hinge, 9512: bearing, 9600: digital signage, 9601: displayportion, 9602: housing, 9603: speaker.

This application is based on Japanese Patent Application serial no.2016-057718 filed with Japan Patent Office on Mar. 22, 2016, JapanesePatent Application serial no. 2016-057720 filed with Japan Patent Officeon Mar. 22, 2016, and Japanese Patent Application serial no. 2016-057716filed with Japan Patent Office on Mar. 22, 2016, the entire contents ofwhich are hereby incorporated by reference.

The invention claimed is:
 1. A transistor comprising: an insulatingfilm; a source electrode; a drain electrode; a gate electrode; and anoxide semiconductor film, wherein the source electrode includes a regionin contact with the oxide semiconductor film, wherein the drainelectrode includes a region in contact with the oxide semiconductorfilm, wherein the gate electrode includes a region that overlaps withthe oxide semiconductor film, wherein the insulating film is providedbetween the gate electrode and the oxide semiconductor film, whereindrain current-gate voltage characteristics of the transistor when asource voltage is 0V, a drain voltage is 20V, and a gate voltage ishigher than −10 V and lower than or equal to 10 V represents: a maximumvalue of field-effect mobility of the transistor is larger than or equalto 40 cm²/Vs and smaller than 150 cm²/Vs; a threshold voltage of thetransistor is higher than or equal to −1 V and lower than or equal to 1V; an S value of the transistor is smaller than 0.3 V/decade; and anoff-state current of the transistor is lower than 1×10⁻¹² A/cm², whereinthe oxide semiconductor film includes a composite oxide semiconductor inwhich a first region and a second region are mixed, wherein the firstregion includes a plurality of first clusters including, as a maincomponent, one or more selected from indium, zinc, and oxygen, whereinthe second region includes a plurality of second clusters including, asa main component, one or more selected from indium, an element M, zinc,and oxygen, wherein the element M is one of Al, Ga, Y, and Sn, whereinthe first region includes a portion in which the plurality of firstclusters are connected to each other, and wherein the second regionincludes a portion in which the plurality of second clusters areconnected to each other.
 2. The transistor according to claim 1, whereinμ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to 1 and smallerthan 1.5, where μ_(FE)(max) represents the maximum value of thefield-effect mobility of the transistor and μ_(FE)(V_(g)=2V) representsa value of the field-effect mobility of the transistor at a gate voltageof 2 V.
 3. The transistor according to claim 1, whereinμ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to 1.5 and smallerthan 3, where μ_(FE)(max) represents the maximum value of thefield-effect mobility of the transistor and μ_(FE)(V_(g)=2V) representsa value of the field-effect mobility of the transistor at a gate voltageof 2 V.
 4. The transistor according to claim 1, wherein an atomic ratioof indium to the element M and zinc is [In]:[M]:[Zn]=4:2:3 or in aneighborhood of 4:2:3 in the second region, and wherein, when the [In]is 4, the [M] is greater than or equal to 1.5 and less than or equal to2.5 and the [Zn] is greater than or equal to 2 and less than or equal to4.
 5. The transistor according to claim 1, wherein an atomic ratio ofindium to the element M and zinc is [In]:[M]:[Zn]=5:1:6 or in aneighborhood of 5:1:6 in the second region, and wherein, when the [In]is 5, the [M] is greater than or equal to 0.5 and less than or equal to1.5 and the [Zn] is greater than or equal to 5 and less than or equal to7.
 6. A display device comprising a display portion comprising a pixel,wherein the pixel comprises the transistor according to claim
 1. 7. Adisplay device comprising a display portion and a driver circuit,wherein the driver circuit comprises the transistor according toclaim
 1. 8. A transistor comprising: a first gate electrode; a firstinsulating film over the first gate electrode; an oxide semiconductorfilm over the first insulating film, a second insulating film over theoxide semiconductor film; a second gate electrode over the secondinsulating film; and a third insulating film over the second gateelectrode, wherein the oxide semiconductor film comprises a channelregion overlapping with the first gate electrode, a source region incontact with the third insulating film, and a drain region in contactwith the third insulating film, wherein the first gate electrode iselectrically connected to the second gate electrode, wherein draincurrent-gate voltage characteristics of the transistor when a sourcevoltage is 0V, a drain voltage is 20V, and a gate voltage is higher than−10 V and lower than or equal to 10 V represents: a maximum value offield-effect mobility of the transistor is larger than or equal to 40cm²/Vs and smaller than 150 cm²/Vs; a threshold voltage of thetransistor is higher than or equal to −1 V and lower than or equal to 1V; an S value of the transistor is smaller than 0.3 V/decade; and anoff-state current of the transistor is lower than 1×10⁻¹² A/cm², whereinthe oxide semiconductor film includes a composite oxide semiconductor inwhich a first region and a second region are mixed, wherein the firstregion includes a plurality of first clusters including, as a maincomponent, one or more selected from indium, zinc, and oxygen, whereinthe second region includes a plurality of second clusters including, asa main component, one or more selected from indium, an element M, zinc,and oxygen, wherein the element M is one of Al, Ga, Y, and Sn, whereinthe first region includes a portion in which the plurality of firstclusters are connected to each other, and wherein the second regionincludes a portion in which the plurality of second clusters areconnected to each other.
 9. The transistor according to claim 8, whereinμ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to 1 and smallerthan 1.5, where μ_(FE)(max) represents the maximum value of thefield-effect mobility of the transistor and μ_(FE)(V_(g)=2V) representsa value of the field-effect mobility of the transistor at a gate voltageof 2 V.
 10. The transistor according to claim 8, whereinμ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equal to 1.5 and smallerthan 3, where μ_(FE)(max) represents the maximum value of thefield-effect mobility of the transistor and μ_(FE)(V_(g)=2V) representsa value of the field-effect mobility of the transistor at a gate voltageof 2 V.
 11. The transistor according to claim 8, wherein an atomic ratioof indium to the element M and zinc is [In]:[M]:[Zn]=4:2:3 or in aneighborhood of 4:2:3 in the second region, and wherein, when the [In]is 4, the [M] is greater than or equal to 1.5 and less than or equal to2.5 and the [Zn] is greater than or equal to 2 and less than or equal to4.
 12. The transistor according to claim 8, wherein an atomic ratio ofindium to the element M and zinc is [In]:[M]:[Zn]=5:1:6 or in aneighborhood of 5:1:6 in the second region, and wherein, when the [In]is 5, the [M] is greater than or equal to 0.5 and less than or equal to1.5 and the [Zn] is greater than or equal to 5 and less than or equal to7.
 13. A display device comprising a display portion comprising a pixel,wherein the pixel comprises the transistor according to claim
 8. 14. Adisplay device comprising a display portion and a driver circuit,wherein the driver circuit comprises the transistor according to claim8.
 15. A transistor comprising: an insulating film; a source electrode;a drain electrode; a gate electrode; and an oxide semiconductor film,wherein the source electrode includes a region in contact with the oxidesemiconductor film, wherein the drain electrode includes a region incontact with the oxide semiconductor film, wherein the gate electrodeincludes a region that overlaps with the oxide semiconductor film,wherein the insulating film is provided between the gate electrode andthe oxide semiconductor film, wherein drain current-gate voltagecharacteristics of the transistor when a source voltage is 0V, a drainvoltage is 20V, and a gate voltage is higher than −10 V and lower thanor equal to 10 V represents: a maximum value of field-effect mobility ofthe transistor is larger than or equal to 10 cm²/Vs and smaller than 100cm²/Vs; a threshold voltage of the transistor is higher than or equal to−1 V and lower than or equal to 1 V; an S value of the transistor issmaller than 0.3 V/decade; and an off-state current of the transistor islower than 1×10⁻¹² A/cm², wherein the oxide semiconductor film includesa composite oxide semiconductor in which a first region and a secondregion are mixed, wherein the first region includes a plurality of firstclusters including, as a main component, one or more selected fromindium, zinc, and oxygen, wherein the second region includes a pluralityof second clusters including, as a main component, one or more selectedfrom indium, an element M, zinc, and oxygen, wherein the element M isone of Al, Ga, Y, and Sn, wherein the first region includes a portion inwhich the plurality of first clusters are connected to each other, andwherein the second region includes a portion in which the plurality ofsecond clusters are connected to each other.
 16. The transistoraccording to claim 15, wherein μ_(FE)(max)/μ_(FE)(V_(g)=2V) is largerthan or equal to 3 and smaller than 10, where μ_(FE)(max) represents themaximum value of the field-effect mobility of the transistor andμ_(FE)(V_(g)=2V) represents a value of the field-effect mobility of thetransistor at a gate voltage of 2 V.
 17. The transistor according toclaim 15, wherein an atomic ratio of indium to the element M and zinc is[In]:[M]:[Zn]=4:2:3 or in a neighborhood of 4:2:3 in the second region,and wherein, when the [In] is 4, the [M] is greater than or equal to 1.5and less than or equal to 2.5 and the [Zn] is greater than or equal to 2and less than or equal to
 4. 18. The transistor according to claim 15,wherein an atomic ratio of indium to the element M and zinc is[In]:[M]:[Zn]=5:1:6 or in a neighborhood of 5:1:6 in the second region,and wherein, when the [In] is 5, the [M] is greater than or equal to 0.5and less than or equal to 1.5 and the [Zn] is greater than or equal to 5and less than or equal to
 7. 19. A display device comprising a displayportion comprising a pixel, wherein the pixel comprises the transistoraccording to claim
 15. 20. A display device comprising a display portionand a driver circuit, wherein the driver circuit comprises thetransistor according to claim
 15. 21. A transistor comprising: a firstgate electrode; a first insulating film over the first gate electrode;an oxide semiconductor film over the first insulating film, a secondinsulating film over the oxide semiconductor film; a second gateelectrode over the second insulating film; and a third insulating filmover the second gate electrode, wherein the oxide semiconductor filmcomprises a channel region overlapping with the first gate electrode, asource region in contact with the third insulating film, and a drainregion in contact with the third insulating film, wherein the first gateelectrode is electrically connected to the second gate electrode,wherein drain current-gate voltage characteristics of the transistorwhen a source voltage is 0V, a drain voltage is 20V, and a gate voltageis higher than −10 V and lower than or equal to 10 V represents: amaximum value of field-effect mobility of the transistor is larger thanor equal to 10 cm²/Vs and smaller than 100 cm²/Vs; a threshold voltageof the transistor is higher than or equal to −1 V and lower than orequal to 1 V; an S value of the transistor is smaller than 0.3 V/decade;and an off-state current of the transistor is lower than 1×10⁻¹² A/cm²,wherein the oxide semiconductor film includes a composite oxidesemiconductor in which a first region and a second region are mixed,wherein the first region includes a plurality of first clustersincluding, as a main component, one or more selected from indium, zinc,and oxygen, wherein the second region includes a plurality of secondclusters including, as a main component, one or more selected fromindium, an element M, zinc, and oxygen, wherein the element M is one ofAl, Ga, Y, and Sn, wherein the first region includes a portion in whichthe plurality of first clusters are connected to each other, and whereinthe second region includes a portion in which the plurality of secondclusters are connected to each other.
 22. The transistor according toclaim 21, wherein μ_(FE)(max)/μ_(FE)(V_(g)=2V) is larger than or equalto 3 and smaller than 10, where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2V)represents a value of the field-effect mobility of the transistor at agate voltage of 2 V.
 23. The transistor according to claim 21, whereinan atomic ratio of indium to the element M and zinc is[In]:[M]:[Zn]=4:2:3 or in a neighborhood of 4:2:3 in the second region,and wherein, when the [In] is 4, the [M] is greater than or equal to 1.5and less than or equal to 2.5 and the [Zn] is greater than or equal to 2and less than or equal to
 4. 24. The transistor according to claim 21,wherein an atomic ratio of indium to the element M and zinc is[In]:[M]:[Zn]=5:1:6 or in a neighborhood of 5:1:6 in the second region,and wherein, when the [In] is 5, the [M] is greater than or equal to 0.5and less than or equal to 1.5 and the [Zn] is greater than or equal to 5and less than or equal to
 7. 25. A display device comprising a displayportion comprising a pixel, wherein the pixel comprises the transistoraccording to claim
 21. 26. A display device comprising a display portionand a driver circuit, wherein the driver circuit comprises thetransistor according to claim 21.